doc: document `afterexcept` dependency scheme #3301

Job	Run time
Generate build matrix	9s
python linting	32s
spelling	6s
validate commits	0s
bookworm - test-install	12m 37s
bookworm - 32 bit	13m 23s
noble - test-install	12m 58s
el9 - test-install	12m 51s
alpine - test-install	13m 46s
fedora40 - test-install	12m 19s
jammy - test-install	12m 6s
focal - py3.8	16m 53s
bookworm - gcc-12,content-s3,distcheck	19m 46s
fedora40 - clang-18	17m 14s
coverage	24m 29s
el8 - ascii	19m 41s
el8 - test-install	14m 8s
el8 - system,coverage	7m 19s
inception	21m 28s
flux-sched check	6m 54s
flux-pmix check	12m 58s
macos build only	4m 21s
flux-accounting check	4m 40s
flux-pam check	4m 18s
Generate docker manifest	1s
	4h 24m 57s

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