sql: Document which sql_modes are supported

[morgo]

In pingcap/tidb#2738 it mentions that TiDB should support more sql_modes, but it is not clear which existing modes are supported.

Because TiDB does not produce an error when setting an unsupported sql_mode, it is important that this be documented on sql/mysql-compatibility.md

[morgo]

An example showing the sql_mode of NO_ENGINE_SUBSTITUTION having no impact:

mysql> SELECT @@sql_mode;
+--------------------------------------------+
| @@sql_mode                                 |
+--------------------------------------------+
| STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+--------------------------------------------+
1 row in set (0.00 sec)

mysql> CREATE TABLE t (a int) engine=idontexist;
Query OK, 0 rows affected (0.18 sec)

[morgo]

Via the mysql docs. Here is my understanding so far:

Fully Supported

• strict-trans-tables
• ansi-quotes
• no-auto-create-user (will merge soon)
• strict-all-tables
• NO_ZERO_IN_DATE
• No_ZERO_DATE
• PIPES_AS_CONCAT
• error-for-division-by-zero
• high-not-precedence
• PAD_CHAR_TO_FULL_LENGTH
• ignore space
• no-auto-value-on-zero
• no backslash escapes
• no unsigned subtraction
• REAL_AS_FLOAT

Partially supported

• only-full-group-by (* minor semantic differences)
• ansi (a combination mode; semantic diffs in only-full-group-by)
• traditional (does not support no engine-substitution)

Not applicable

• no-dir-in-create
• no-engine-substitution

Deprecated, will not add

• no field options
• no key options
• no table options
• db2
• maxdb
• mssql
• MYSQL323
• mysql40
• oracle
• POSTGRESQL

unsupported

• allow-invalid-dates ( sql_mode ALLOW_INVALID_DATES is not supported tidb#8263 )