*: update "change column" and "begin"

sql-statements/sql-statement-add-index.md

@@ -67,6 +67,7 @@ mysql> EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
 ## MySQL compatibility
 
 * `FULLTEXT`, `HASH` and `SPATIAL` indexes are not supported.
+* `VISIBLE/INVISIBLE` index is not supported (currently only the master branch actually supports this feature).
 * Descending indexes are not supported (similar to MySQL 5.7).
 * Adding multiple indexes at the same time is currently not supported.
 * Adding the primary key constraint to a table is not supported by default. You can enable the feature by setting the `alter-primary-key` configuration item to `true`. For details, see [alter-primary-key](/tidb-configuration-file.md#alter-primary-key).

sql-statements/sql-statement-begin.md

@@ -35,10 +35,12 @@ Query OK, 0 rows affected (0.01 sec)
 
 ## MySQL compatibility
 
-This statement is understood to be fully compatible with MySQL. Any compatibility differences should be [reported via an issue](/report-issue.md) on GitHub.
+You can add the `PESSIMISTIC` or `OPTIMISTIC` option to the `BEGIN` statement in TiDB to indicate the type of transaction that this statement starts.
 
 ## See also
 
 * [COMMIT](/sql-statements/sql-statement-commit.md)
 * [ROLLBACK](/sql-statements/sql-statement-rollback.md)
 * [START TRANSACTION](/sql-statements/sql-statement-start-transaction.md)
+* [TiDB optimistic transaction model](/optimistic-transaction.md)
+* [TiDB pessimistic transaction model](/pessimistic-transaction.md)

sql-statements/sql-statement-change-column.md

@@ -65,6 +65,8 @@ ERROR 1105 (HY000): can't run multi schema change
 
 * Making multiple changes in a single `ALTER TABLE` statement is not currently supported.
 * Only certain types of data type changes are supported. For example, an `INTEGER` to `BIGINT` is supported, but the reverse is not possible. Changing from an integer to a string format or blob is not supported.
+* Modifying precision of the `DECIMAL` type is not supported.
+* Changing the `UNSIGNED` attribute is not supported.
 
 ## See also
 

sql-statements/sql-statement-commit.md

@@ -17,6 +17,10 @@ In the absence of a `BEGIN` or `START TRANSACTION` statement, the default behavi
 
 ![CommitStmt](/media/sqlgram/CommitStmt.png)
 
+**CompletionTypeWithinTransaction:**
+
+![CompletionTypeWithinTransaction](/media/sqlgram/CompletionTypeWithinTransaction.png)
+
 ## Examples
 
 ```sql