Auto-inc docs should describe behavior as InnoDB-like

Error Report

Please answer the following questions before submitting your issue. Thanks!

1. What is the URL/path of the document related to this issue?

https://docs.pingcap.com/tidb/stable/auto-increment#restrictions

2. How would you like to improve it?

The behavior of auto-increment is engine specific in MySQL. Although the mysql-compatibility page already documents that engine=XX is ignored, it is worth adding a note to the auto-increment page under "restrictions" to state this.

Here is an example to show the behavior difference:

CREATE TABLE animals (
    grp ENUM('fish','mammal','bird') NOT NULL,
    id MEDIUMINT NOT NULL AUTO_INCREMENT,
    name CHAR(30) NOT NULL,
    PRIMARY KEY (grp,id)
) ENGINE=MyISAM;
INSERT INTO animals (grp,name) VALUES
    ('mammal','dog'),('mammal','cat'),
    ('bird','penguin'),('fish','lax'),('mammal','whale'),
    ('bird','ostrich');
SELECT * FROM animals ORDER BY grp,id;

Start with the linked auto-increment documentation and its Restrictions section, then compare the existing mysql-compatibility page and the supplied SQL example. Done means the auto-increment page clearly notes the engine-specific behavior and its relationship to TiDB's compatibility behavior.