Improve description of buffer used to store records in WAL reader

The dedicated private buffer to store records is used only for these
crossing a page boundary since 285bd0ac, but its description did not
match completely the reality.

Reported-by: Andrey Lepikhov
Author: Michael Paquier
Discussion: https://postgr.es/m/49518b48-2036-5e43-1818-0f594e375e76@postgrespro.ru

src/include/access/xlogreader.h

@@ -185,7 +185,10 @@ struct XLogReaderState
 	 */
 	TimeLineID	nextTLI;
 
-	/* Buffer for current ReadRecord result (expandable) */
+	/*
+	 * Buffer for current ReadRecord result (expandable), used when a record
+	 * crosses a page boundary.
+	 */
 	char	   *readRecordBuf;
 	uint32		readRecordBufSize;