Clean up some GUC declarations and comments

This adjusts a few things for GUCs related to logical replication,
replication slots and WAL senders, in the shape of incorrect comments
and values inconsistent with their initial default value.

Author: Peter Smith
Reviewed-by: Nathan Bossart, Tom Lane, Justin Pryzby
Discussion: https://postgr.es/m/CAHut+PtHE0XSfjjRQ6D4v7+dqzCw=d+1a64ujra4EX8aoc_Z+w@mail.gmail.com

src/backend/replication/logical/launcher.c

@@ -52,6 +52,7 @@
 /* max sleep time between cycles (3min) */
 #define DEFAULT_NAPTIME_PER_CYCLE 180000L
 
+/* GUC variables */
 int			max_logical_replication_workers = 4;
 int			max_sync_workers_per_subscription = 2;
 

src/backend/replication/slot.c

@@ -97,8 +97,8 @@ ReplicationSlotCtlData *ReplicationSlotCtl = NULL;
 /* My backend's replication slot in the shared memory array */
 ReplicationSlot *MyReplicationSlot = NULL;
 
-/* GUCs */
-int			max_replication_slots = 0;	/* the maximum number of replication
+/* GUC variable */
+int			max_replication_slots = 10; /* the maximum number of replication
 										 * slots */
 
 static void ReplicationSlotShmemExit(int code, Datum arg);

src/backend/replication/walsender.c

@@ -118,8 +118,8 @@ bool		am_cascading_walsender = false; /* Am I cascading WAL to another
 											 * standby? */
 bool		am_db_walsender = false;	/* Connected to a database? */
 
-/* User-settable parameters for walsender */
-int			max_wal_senders = 0;	/* the maximum number of concurrent
+/* GUC variables */
+int			max_wal_senders = 10;	/* the maximum number of concurrent
 									 * walsenders */
 int			wal_sender_timeout = 60 * 1000; /* maximum time to send one WAL
 											 * data message */