Fix comments around X.509/RFC5280 99991231235959Z time value

[msandstedt]

Problem

X.509/RFC5280 defines the special time 99991231235959Z to mean 'no
well-defined expiration date'. Comments in the code implied this was
also relevant for NotBefore, but it's not. This is only a NotAfter
concept.

Change overview

In spite of the misleading comments, the code does have reasonable behavior for both the NotBefore and NotAfter cases. This commit amends comments to spell out exactly what this behavior is.

Testing

Tested to build without error.

[github-actions]

PR #19572: Size comparison from 4b6fc80 to dab1852

Increases (3 builds for cc13x2_26x2, esp32, nrfconnect)

platform	target	config	section	4b6fc80	dab1852	change	% change
cc13x2_26x2	lock-mtd	LP_CC2652R7	(read only)	637939	637947	8	0.0
			.text	538684	538692	8	0.0
esp32	all-clusters-app	c3devkit	(read only)	1013900	1013902	2	0.0
			.flash.text	1013900	1013902	2	0.0
nrfconnect	all-clusters-minimal-app	nrf52840dk_nrf52840	text	792752	792756	4	0.0

Decreases (1 build for cyw30739)

platform	target	config	section	4b6fc80	dab1852	change	% change
cyw30739	lock	cyw930739m2evb_01	(read/write)	609318	609310	-8	-0.0
			.app_xip_area	464808	464800	-8	-0.0

Full report (30 builds for cc13x2_26x2, cyw30739, efr32, esp32, k32w, linux, mbed, nrfconnect, p6, telink)

platform	target	config	section	4b6fc80	dab1852	change	% change
cc13x2_26x2	all-clusters-app	LP_CC2652R7	(read only)	659119	659119	0	0.0
			(read/write)	191784	191784	0	0.0
			.bss	73796	73796	0	0.0
			.data	3416	3416	0	0.0
			.rodata	87047	87047	0	0.0
			.text	571760	571760	0	0.0
	all-clusters-minimal-app	LP_CC2652R7	(read only)	645667	645667	0	0.0
			(read/write)	157292	157292	0	0.0
			.bss	73020	73020	0	0.0
			.data	3356	3356	0	0.0
			.rodata	90067	90067	0	0.0
			.text	555280	555280	0	0.0
	lock-ftd	LP_CC2652R7	(read only)	688531	688531	0	0.0
			(read/write)	153444	153444	0	0.0
			.bss	71756	71756	0	0.0
			.data	3280	3280	0	0.0
			.rodata	98883	98883	0	0.0
			.text	589164	589164	0	0.0
	lock-mtd	LP_CC2652R7	(read only)	637939	637947	8	0.0
			(read/write)	144888	144888	0	0.0
			.bss	67492	67492	0	0.0
			.data	3280	3280	0	0.0
			.rodata	98763	98763	0	0.0
			.text	538684	538692	8	0.0
	pump-app	LP_CC2652R7	(read only)	669747	669747	0	0.0
			(read/write)	173124	173124	0	0.0
			.bss	71884	71884	0	0.0
			.data	3316	3316	0	0.0
			.rodata	86979	86979	0	0.0
			.text	582284	582284	0	0.0
	pump-controller-app	LP_CC2652R7	(read only)	660471	660471	0	0.0
			(read/write)	182520	182520	0	0.0
			.bss	72004	72004	0	0.0
			.data	3276	3276	0	0.0
			.rodata	84903	84903	0	0.0
			.text	575084	575084	0	0.0
	shell	LP_CC2652R7	(read only)	689270	689270	0	0.0
			(read/write)	157128	157128	0	0.0
			.bss	76092	76092	0	0.0
			.data	3420	3420	0	0.0
			.rodata	110126	110126	0	0.0
			.text	578832	578832	0	0.0
cyw30739	light	cyw930739m2evb_01	(read/write)	612202	612202	0	0.0
			.app_xip_area	467836	467836	0	0.0
			.bss	87288	87288	0	0.0
			.data	764	764	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
	lock	cyw930739m2evb_01	(read/write)	609318	609310	-8	-0.0
			.app_xip_area	464808	464800	-8	-0.0
			.bss	87464	87464	0	0.0
			.data	732	732	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
	ota-requestor-no-progress-logging	cyw930739m2evb_01	(read/write)	615382	615382	0	0.0
			.app_xip_area	471928	471928	0	0.0
			.bss	86464	86464	0	0.0
			.data	672	672	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
efr32	lighting-app	BRD4161A	(read only)	920584	920584	0	0.0
			(read/write)	133456	133456	0	0.0
			.bss	131336	131336	0	0.0
			.data	2116	2116	0	0.0
			.text	920576	920576	0	0.0
		BRD4161A+rpc	(read only)	956240	956240	0	0.0
			(read/write)	150328	150328	0	0.0
			.bss	148008	148008	0	0.0
			.data	2320	2320	0	0.0
			.text	956232	956232	0	0.0
		BRD4161A+rs911x	(read only)	795700	795700	0	0.0
			(read/write)	129736	129736	0	0.0
			.bss	127612	127612	0	0.0
			.data	2124	2124	0	0.0
			.text	795692	795692	0	0.0
	lock-app	BRD4161A+wf200	(read only)	963596	963596	0	0.0
			(read/write)	130076	130076	0	0.0
			.bss	127988	127988	0	0.0
			.data	2088	2088	0	0.0
			.text	963588	963588	0	0.0
	window-app	BRD4161A	(read only)	905472	905472	0	0.0
			(read/write)	133528	133528	0	0.0
			.bss	131416	131416	0	0.0
			.data	2108	2108	0	0.0
			.text	905464	905464	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	1013900	1013902	2	0.0
			(read/write)	1483522	1483522	0	0.0
			.dram0.bss	69424	69424	0	0.0
			.dram0.data	14696	14696	0	0.0
			.flash.rodata	213968	213968	0	0.0
			.flash.text	1013900	1013902	2	0.0
			.iram0.text	62902	62902	0	0.0
		m5stack	(read only)	1068115	1068115	0	0.0
			(read/write)	485616	485616	0	0.0
			.dram0.bss	74952	74952	0	0.0
			.dram0.data	34224	34224	0	0.0
			.flash.rodata	244444	244444	0	0.0
			.flash.text	1062731	1062731	0	0.0
			.iram0.text	123267	123267	0	0.0
k32w	light	k32w061+release	(read/write)	658328	658328	0	0.0
			.bss	69764	69764	0	0.0
			.data	2028	2028	0	0.0
			.text	580736	580736	0	0.0
	lock	k32w061+release	(read/write)	719940	719940	0	0.0
			.bss	70196	70196	0	0.0
			.data	2000	2000	0	0.0
			.text	641944	641944	0	0.0
linux	chip-tool-no-interactive-ipv6only	arm64	(read only)	9580764	9580764	0	0.0
			(read/write)	690049	690049	0	0.0
			.bss	43697	43697	0	0.0
			.data	1152	1152	0	0.0
			.data.rel.ro	626288	626288	0	0.0
			.dynamic	528	528	0	0.0
			.got	15104	15104	0	0.0
			.init	24	24	0	0.0
			.init_array	192	192	0	0.0
			.rodata	470924	470924	0	0.0
			.text	7533236	7533236	0	0.0
	thermostat-no-ble	arm64	(read only)	2560628	2560628	0	0.0
			(read/write)	191489	191489	0	0.0
			.bss	99393	99393	0	0.0
			.data	1560	1560	0	0.0
			.data.rel.ro	82432	82432	0	0.0
			.dynamic	528	528	0	0.0
			.got	5080	5080	0	0.0
			.init	24	24	0	0.0
			.init_array	400	400	0	0.0
			.rodata	161356	161356	0	0.0
			.text	2161472	2161472	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2434816	2434816	0	0.0
			.bss	209212	209212	0	0.0
			.data	5864	5864	0	0.0
			.text	1397460	1397460	0	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	(read/write)	1199111	1199111	0	0.0
			bss	141633	141633	0	0.0
			rodata	156128	156128	0	0.0
			text	822412	822412	0	0.0
	all-clusters-minimal-app	nrf52840dk_nrf52840	(read/write)	1144347	1144347	0	0.0
			bss	140824	140824	0	0.0
			rodata	131860	131860	0	0.0
			text	792752	792756	4	0.0
p6	all-clusters-app	default	(read/write)	2552984	2552984	0	0.0
			.bss	143424	143424	0	0.0
			.data	2832	2832	0	0.0
			.text	1511248	1511248	0	0.0
	all-clusters-minimal-app	default	(read/write)	2495472	2495472	0	0.0
			.bss	142640	142640	0	0.0
			.data	2776	2776	0	0.0
			.text	1453736	1453736	0	0.0
	light-app	default	(read/write)	2426848	2426848	0	0.0
			.bss	135752	135752	0	0.0
			.data	2624	2624	0	0.0
			.text	1385112	1385112	0	0.0
	lock-app	default	(read/write)	2447256	2447256	0	0.0
			.bss	135576	135576	0	0.0
			.data	2600	2600	0	0.0
			.text	1405520	1405520	0	0.0
telink	light-switch-app	tlsr9518adk80d	(read/write)	787840	787840	0	0.0
			bss	70892	70892	0	0.0
			noinit	40416	40416	0	0.0
			text	557712	557712	0	0.0
	lighting-app	tlsr9518adk80d	(read/write)	807828	807828	0	0.0
			bss	71144	71144	0	0.0
			noinit	40416	40416	0	0.0
			text	574404	574404	0	0.0