Update documentation for regenerating a single app's files.

[bzbarsky-apple]

Also adds logging to zap_regen_all.py so it shows exactly what command
it's running for every target it generates.

Problem

Docs are out of sync with recent code changes.

Change overview

Sync up the docs.

Testing

Ran zap_regen_all and observed the logging and that it still generates what needs to be generated.

[github-actions]

Size increase report for "esp32-example-build" from 6525df0

File	Section	File	VM
chip-lock-app.elf	.flash.text	68	68
chip-shell.elf	.flash.text	56	56

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock-app.elf and ./pull_artifact/chip-lock-app.elf:

sections,vmsize,filesize
.flash.text,68,68
[Unmapped],0,-68

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.flash.text,56,56
[Unmapped],0,-56

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-bridge-app.elf and ./pull_artifact/chip-bridge-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-ipv6only-app.elf and ./pull_artifact/chip-ipv6only-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize

[woody-apple]

@saurabhst @msandstedt @mspang @andy31415 @Damian-Nordic @LuDuda ?