Fix outdated comment

Commit bc971f4 replaced the latch-setting mechanism that the
comment talked about with a condition variable. And before that,
commit 2258e76 moved the code so that the comment got detached from
the loop that it talked about, so move the comment closer to the loop.

Author: hlinnaka

src/backend/access/transam/xlogutils.c

@@ -840,11 +840,6 @@ wal_segment_close(XLogReaderState *state)
  *
  * Public because it would likely be very helpful for someone writing another
  * output method outside walsender, e.g. in a bgworker.
- *
- * TODO: The walsender has its own version of this, but it relies on the
- * walsender's latch being set whenever WAL is flushed. No such infrastructure
- * exists for normal backends, so we have to do a check/sleep/repeat style of
- * loop for now.
  */
 int
 read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
@@ -884,7 +879,14 @@ read_local_xlog_page_guts(XLogReaderState *state, XLogRecPtr targetPagePtr,
 
 	loc = targetPagePtr + reqLen;
 
-	/* Loop waiting for xlog to be available if necessary */
+	/*
+	 * Loop waiting for xlog to be available if necessary
+	 *
+	 * TODO: The walsender has its own version of this function, which uses a
+	 * condition variable to wake up whenever WAL is flushed. We could use the
+	 * same infrastructure here, instead of the check/sleep/repeat style of
+	 * loop.
+	 */
 	while (1)
 	{
 		/*