[PATCH] Update HandleChildCrash comments

Hello hackers,

The existing header comments for HandleChildCrash() in postmaster.c
are somewhat outdated,
as they only list a few auxiliary processes and miss several newer
ones added in recent versions, for example: walsummarizer, and
io_worker.

Instead of maintaining an exhaustive list of process names in the
comments, which can become stale.
This patch updates the comments to point directly to the
decision-making places: process_pm_child_exit() and CleanupBackend().

I have preserved the original description regarding the function's
objectives (cleaning up local state and signaling other children).

Please see the attached patch.

--
Thanks,
Tender Wang