Fix: Process::Status#exit_code doc (#15351) #23

Job	Run time
x86_64-gnu-test (1.7.3)	33m 42s
check_format	1m 39s
x86_64-gnu-test (1.8.2)	35m 14s
x86_64-gnu-test-preview_mt	5m 45s
x86_64-gnu-test (1.9.2)	34m 19s
x86_64-musl-test	37m 45s
x86_64-gnu-test (1.10.1)	35m 10s
x86_64-gnu-test (1.11.2)	36m 25s
x86_64-gnu-test (1.12.2)	36m 26s
x86_64-gnu-test (1.13.3)	32m 49s
x86_64-gnu-test (1.14.1)	36m 35s
x86_64-gnu-test (1.15.0)	34m 21s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 15s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 7s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	33m 34s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 11s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	33m 8s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 25s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	33m 6s
	9h 55m 56s

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