Fix comment in xlogutils.c

The existing callers of XLogReadDetermineTimeline() performing recovery
need to check a replay LSN position when determining on which timeline
to read a WAL page.  A portion of the comment describing this function
said exactly that, while referring to a routine for fetching a write
LSN, something not available in recovery.

Author: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20200511.101619.2043820539323292957.horikyota.ntt@gmail.com

src/backend/access/transam/xlogutils.c

@@ -681,10 +681,10 @@ XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
  * copied to a new timeline.
  *
  * The caller must also make sure it doesn't read past the current replay
- * position (using GetWalRcvWriteRecPtr) if executing in recovery, so it
+ * position (using GetXLogReplayRecPtr) if executing in recovery, so it
  * doesn't fail to notice that the current timeline became historical. The
  * caller must also update ThisTimeLineID with the result of
- * GetWalRcvWriteRecPtr and must check RecoveryInProgress().
+ * GetXLogReplayRecPtr and must check RecoveryInProgress().
  */
 void
 XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wantLength)