Fix misleading comments about background worker registration.

Since 6bc8ef0b7f, the maximum number
of backends can't change as background workers are registered, but
these comments still reflect the way things worked prior to that.

Also, per recent discussion, some modules call SetConfigOption()
from _PG_init(). It's not entirely clear to me whether we want to
regard that as a fully supported operation, but since we know it's
a thing that happens, it at least deserves a mention in the comments,
so add that.

Nathan Bossart, reviewed by Anton A. Melnikov

Discussion: http://postgr.es/m/20220419154658.GA2487941@nathanxps13

src/backend/utils/init/postinit.c

@@ -538,9 +538,8 @@ pg_split_opts(char **argv, int *argcp, const char *optstr)
 /*
  * Initialize MaxBackends value from config options.
  *
- * This must be called after modules have had the chance to register background
- * workers in shared_preload_libraries, and before shared memory size is
- * determined.
+ * This must be called after modules have had the chance to alter GUCs in
+ * shared_preload_libraries and before shared memory size is determined.
  *
  * Note that in EXEC_BACKEND environment, the value is passed down from
  * postmaster to subprocesses via BackendParameters in SubPostmasterMain; only