Fix comments on row_merge_write()

This is a port of
vasil.dimov@oracle.com-20100521175337-c1b1lqxgizqegb0w and
vasil.dimov@oracle.com-20100521180951-mef23h24k023xuwq
from mysql-trunk-innodb

storage/innodb_plugin/row/row0merge.c

@@ -717,14 +717,16 @@ row_merge_read(
 }
 
 /********************************************************************//**
-Read a merge block from the file system.
+Write a merge block to the file system.
 @return	TRUE if request was successful, FALSE if fail */
 static
 ibool
 row_merge_write(
 /*============*/
 	int		fd,	/*!< in: file descriptor */
-	ulint		offset,	/*!< in: offset where to write */
+	ulint		offset,	/*!< in: offset where to read
+				in number of row_merge_block_t
+				elements */
 	const void*	buf)	/*!< in: data */
 {
 	ib_uint64_t	ofs = ((ib_uint64_t) offset)