Improve comments and logs in do_pg_stop/start_backup

The function name pg_stop_backup() has been included for ages in some
log messages when stopping the backup, which is confusing for base
backups taken with the replication protocol because this function is
never called.  Some other comments and messages in this area are
improved while on it.

The new wording is based on input and suggestions from several people,
all listed below.

Author: Michael Paquier
Reviewed-by: Peter Eisentraut, Álvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/20181221040510.GA12599@paquier.xyz

src/backend/access/transam/xlog.c

@@ -10170,9 +10170,10 @@ XLogFileNameP(TimeLineID tli, XLogSegNo segno)
 }
 
 /*
- * do_pg_start_backup is the workhorse of the user-visible pg_start_backup()
- * function. It creates the necessary starting checkpoint and constructs the
- * backup label file.
+ * do_pg_start_backup
+ *
+ * Utility function called at the start of an online backup. It creates the
+ * necessary starting checkpoint and constructs the backup label file.
  *
  * There are two kind of backups: exclusive and non-exclusive. An exclusive
  * backup is started with pg_start_backup(), and there can be only one active
@@ -10712,8 +10713,10 @@ get_backup_status(void)
 }
 
 /*
- * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
- * function.
+ * do_pg_stop_backup
+ *
+ * Utility function called at the end of an online backup. It cleans up the
+ * backup state and can optionally wait for WAL segments to be archived.
  *
  * If labelfile is NULL, this stops an exclusive backup. Otherwise this stops
  * the non-exclusive backup specified by 'labelfile'.
@@ -11084,7 +11087,7 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 			if (!reported_waiting && waits > 5)
 			{
 				ereport(NOTICE,
-						(errmsg("pg_stop_backup cleanup done, waiting for required WAL segments to be archived")));
+						(errmsg("base backup done, waiting for required WAL segments to be archived")));
 				reported_waiting = true;
 			}
 
@@ -11094,16 +11097,16 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 			{
 				seconds_before_warning *= 2;	/* This wraps in >10 years... */
 				ereport(WARNING,
-						(errmsg("pg_stop_backup still waiting for all required WAL segments to be archived (%d seconds elapsed)",
+						(errmsg("still waiting for all required WAL segments to be archived (%d seconds elapsed)",
 								waits),
 						 errhint("Check that your archive_command is executing properly.  "
-								 "pg_stop_backup can be canceled safely, "
+								 "You can safely cancel this backup, "
 								 "but the database backup will not be usable without all the WAL segments.")));
 			}
 		}
 
 		ereport(NOTICE,
-				(errmsg("pg_stop_backup complete, all required WAL segments have been archived")));
+				(errmsg("all required WAL segments have been archived")));
 	}
 	else if (waitforarchive)
 		ereport(NOTICE,