Update docs and error message for superuser_reserved_connections.

Commit ea92368cd1 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

doc/src/sgml/config.sgml

@@ -725,8 +725,7 @@ include_dir 'conf.d'
         number of active concurrent connections is at least
         <varname>max_connections</varname> minus
         <varname>superuser_reserved_connections</varname>, new
-        connections will be accepted only for superusers, and no
-        new replication connections will be accepted.
+        connections will be accepted only for superusers.
        </para>
 
        <para>

src/backend/utils/init/postinit.c

@@ -931,7 +931,7 @@ InitPostgres(const char *in_dbname, Oid dboid,
 		!HaveNFreeProcs(ReservedBackends))
 		ereport(FATAL,
 				(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
-				 errmsg("remaining connection slots are reserved for non-replication superuser connections")));
+				 errmsg("remaining connection slots are reserved for superusers")));
 
 	/* Check replication permissions needed for walsender processes. */
 	if (am_walsender)