Fix comment for MAX_BACKENDS.

Fix comment for MAX_BACKENDS.

This comment mentions that we check that the configured number of
backends does not exceed MAX_BACKENDS in RegisterBackgroundWorker()
and relevant GUC check hooks, neither of which has those checks
anymore.  To fix, adjust this comment to say that we do the check
in InitializeMaxBackends().

Oversights in commits 6bc8ef0b7f and 0b1fe1413e.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/Z7zOEzz8lNjaU9yf%40nathan