Fix obsolete references to "XLogRead"

The one in xlogreader.h was pointed out by Antonin Houska; I (Álvaro) noticed the
others by grepping.

Author: Antonin Houska <ah@cybertec.at>
Discussion: https://postgr.es/m/28250.1589186654@antos

src/backend/replication/walsender.c

@@ -932,7 +932,7 @@ CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
 
 	parseCreateReplSlotOptions(cmd, &reserve_wal, &snapshot_action);
 
-	/* setup state for XLogRead */
+	/* setup state for WalSndSegmentOpen */
 	sendTimeLineIsHistoric = false;
 	sendTimeLine = ThisTimeLineID;
 
@@ -2634,7 +2634,7 @@ XLogSendPhysical(void)
 		 *
 		 * Attempt to send all data that's already been written out and
 		 * fsync'd to disk.  We cannot go further than what's been written out
-		 * given the current implementation of XLogRead().  And in any case
+		 * given the current implementation of WALRead().  And in any case
 		 * it's unsafe to send WAL that is not securely down to disk on the
 		 * master: if the master subsequently crashes and restarts, standbys
 		 * must not have applied any WAL that got lost on the master.

src/include/access/xlogreader.h

@@ -103,7 +103,7 @@ typedef struct XLogReaderRoutine
 	 *
 	 * "segcxt" is additional information about the segment.
 	 *
-	 * "tli_p" is an input/output argument. XLogRead() uses it to pass the
+	 * "tli_p" is an input/output argument. WALRead() uses it to pass the
 	 * timeline in which the new segment should be found, but the callback can
 	 * use it to return the TLI that it actually opened.
 	 *