fixup! added syntax reference for ON DUPLICATE KEY clause

docs/sql/reference/insert.txt

@@ -17,9 +17,7 @@ Synopsis
       [ ( column_ident [, ...] ) ]
       { VALUES ( expression [, ...] ) [, ...] | query }
       [ ON DUPLICATE KEY UPDATE
-        { column_ident = expression |
-          ( column_ident [, ...] ) = ( expression [,...] )
-        } [, ...]
+        { column_ident = expression } [, ...]
       ]
 
 Description
@@ -44,22 +42,21 @@ On Duplicate Key Update
 -----------------------
 
 If ``ON DUPLICATE KEY UPDATE`` is specified and a row is inserted that
-would cause a duplicate PRIMARY KEY value, an update of the existing
+would cause a duplicate-key conflict, an update of the existing
 row is performed.
 
 ::
 
-      ON DUPLICATE KEY UPDATE
-      { column_ident = expression |
-        ( column_ident [, ...] ) = ( expression [,...] )
-      } [, ...]
+      ON DUPLICATE KEY UPDATE { column_ident = expression } [, ...]
 
 Within expressions in the ``UPDATE`` clause you can use the
 ``VALUES(column_ident)`` function to refer to column values
 from the INSERT statement.
 So ``VALUES(column_ident)`` in the ``UPDATE`` clause referes to the
 value of the column_ident that would be inserted if no duplicate-key
 conflict occured.
+This function is especially useful in multiple-row inserts, because
+the values of the current row can be referenced.
 
 Parameters
 ==========