Add page on MySQL commands that are noops in TiDB

[morgo]

It might be useful to clarify in one location that some MySQL-isms aren't relevant to TiDB, and so for compatibility just return OK.

For example: FLUSH LOGS, FLUSH HOSTS, etc.

[morgo]

More examples:

• Foreign keys
• Get lock /select lock
• Isolation levels other than repeatable-read
• various sysvars
• collations