mysql-compatibility: add doc for a UPDATE incompatibility case (#7094)

mysql-compatibility.md

@@ -127,6 +127,10 @@ These differences are documented further in [`ANALYZE TABLE`](/sql-statements/sq
 
 For details, see the [`SELECT`](/sql-statements/sql-statement-select.md) statement reference.
 
+### `UPDATE` statement
+
+See the [`UPDATE`](/sql-statements/sql-statement-update.md) statement reference.
+
 ### Views
 
 Views in TiDB are not updatable. They do not support write operations such as `UPDATE`, `INSERT`, and `DELETE`.

sql-statements/sql-statement-update.md

@@ -71,7 +71,17 @@ mysql> SELECT * FROM t1;
 
 ## MySQL compatibility
 
-This statement is understood to be fully compatible with MySQL. Any compatibility differences should be [reported via an issue](https://github.com/pingcap/tidb/issues/new/choose) on GitHub.
+TiDB always uses the original value of a column when evaluating expressions. For example:
+
+```sql
+CREATE TABLE t (a int, b int);
+INSERT INTO t VALUES (1,2);
+UPDATE t SET a = a+1,b=a;
+```
+
+In MySQL, the column `b` is updated to 2 because it is set to the value of `a`, and the value of `a` (which is 1) is updated to `a+1` (which is 2) in the same statement. 
+
+TiDB follows the more standard SQL behavior, and updates `b` to 1.
 
 ## See also