Fix thinko in comment.

WAL segment means a 16 MB physical WAL file; this comment meant a logical
4 GB log file.

Amit Langote. Apply to backbranches only, as the comment is gone in master.

src/backend/access/transam/xlog.c

@@ -3777,7 +3777,7 @@ ReadRecord(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt)
 		if (XLOG_BLCKSZ - (RecPtr->xrecoff % XLOG_BLCKSZ) < SizeOfXLogRecord)
 			NextLogPage(*RecPtr);
 
-		/* Check for crossing of xlog segment boundary */
+		/* Check for crossing of xlog logid boundary */
 		if (RecPtr->xrecoff >= XLogFileSize)
 		{
 			(RecPtr->xlogid)++;