run-command: fix return value comment

run-command: fix return value comment

483bbd4e4c (run-command: introduce child_process_init(), 2014-08-19) and
2d71608ec0 (run-command: factor out child_process_clear(), 2015-10-24)
added help texts about child_process_init() and child_process_clear()
without updating the immediately following documentation of return codes
that only applied to the preexisting functions.

4c4066d95d (run-command: move doc to run-command.h, 2019-11-17) started
to list the functions explicitly that this paragraph applies to, but
still wrongly included child_process_init() and child_process_clear().
Remove their names from that list.

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>