Update docs and error message for superuser_reserved_connections.

Update docs and error message for superuser_reserved_connections.

Commit ea92368cd1da1e290f9ab8efb7f60cb7598fc310 made max_wal_senders
a separate pool of backends from max_connections, but the documentation
and error message for superuser_reserved_connections weren't updated
at the time, and as a result are somewhat misleading. Update.

This is arguably a back-patchable bug fix, but because it seems quite
minor, no back-patch.

Patch by Nathan Bossart. Reviewed by Tushar Ahuja and by me.

Discussion: http://postgr.es/m/20230119194601.GA4105788@nathanxps13