ALTER TABLE t ADD PARTITION needs 'PARTITIONS n' for KEY and HASH partitioning, just a number is not parsed.

Change Request

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

1. Describe what you find is inappropriate or missing in the existing docs.
   In https://docs.pingcap.com/tidb/stable/partitioned-table it says:

Increase the number of partitions using the `ALTER TABLE <table name> ADD PARTITION <number of partitions to increase by | (additional partition definitions)>` statement.

But it needs to be

Increase the number of partitions using the `ALTER TABLE <table name> ADD PARTITION {PARTITIONS <number of partitions to increase by> | (<additional partition definitions>)}` statement.

I.e. ALTER TABLE t ADD PARTITION 2 is not valid syntax, while ALTER TABLE t ADD PARTITION PARTITIONS 2 is.

2. Describe your suggestion or addition.

3. Provide some reference materials (such as documents and websites) if you could.
   https://docs.pingcap.com/tidb/stable/partitioned-table
   https://dev.mysql.com/doc/refman/8.0/en/partitioning-management-hash-key.html

Open the stable/partitioned-table documentation page and compare its ALTER TABLE ... ADD PARTITION syntax with the MySQL partitioning reference linked in the issue. Update the statement to show PARTITIONS before the count for KEY and HASH partitioning, then verify that the examples distinguish this form from additional partition definitions.