doc: tweak description of FLUX_REACTOR_SIGCHLD #2517

Job	Run time
spelling	7s
validate commits	0s
Generate build matrix	7s
python linting	30s
bookworm - test-install	12m 32s
bookworm - 32 bit	13m 11s
noble - test-install	13m 28s
el9 - test-install	12m 38s
alpine - test-install	12m 12s
fedora40 - test-install	1h 0m 31s
jammy - test-install	12m 36s
focal - py3.8	17m 28s
bookworm - gcc-12,content-s3,distcheck	19m 15s
fedora40 - clang-18	17m 8s
coverage	23m 55s
el8 - ascii	20m 10s
el8 - test-install	14m 12s
el8 - system,coverage	7m 34s
inception	22m 28s
flux-pmix check	13m 18s
flux-accounting check	4m 35s
flux-pam check	4m 3s
macos build only	3m 47s
flux-sched check	6m 55s
Generate docker manifest	0s
	5h 12m 40s

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