libsubprocess: add extra documentation #9602

Job	Run time
Generate build matrix	10s
validate commits	9s
python linting	29s
spelling	6s
bookworm - test-install	11m 53s
bookworm - 32 bit	12m 47s
noble - test-install	12m 29s
el9 - test-install	12m 38s
alpine - test-install	12m 7s
fedora40 - test-install	12m 15s
jammy - test-install	11m 21s
focal - py3.8	16m 37s
bookworm - gcc-12,content-s3,distcheck	19m 42s
fedora40 - clang-18	16m 51s
coverage	25m 21s
el8 - ascii	19m 5s
el8 - system,coverage	7m 42s
inception	22m 16s
flux-sched check	6m 35s
flux-pmix check	12m 39s
flux-accounting check	4m 22s
flux-pam check	3m 50s
Generate docker manifest	1s
	4h 1m 25s

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