Add Partition Management as Physical DDL statements

[mjonss]

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/ddl-introduction paragraph "Physical DDL statements" it says:
"Currently, physical DDL statements only include ADD INDEX and lossy column type changes", but Partition management statements like ALTER TABLE t REORGANIZE PARTITION ..., ALTER TABLE t PARTITION BY ... and ALTER TABLE t REMOVE PARTITIONING should also be considered as physical DDL statements, since they will both copy data to new partitions as well as recreate indexes. In addition, if using Global Indexes with partitioned tables, also DROP/TRUNCATE PARTITION will become Physical DDLs.

2. Describe your suggestion or addition.
   "Currently, physical DDL statements include ADD INDEX, some partitioning management statements and lossy column type changes"

3. Provide some reference materials (such as documents and websites) if you could.
   https://docs.pingcap.com/tidb/stable/ddl-introduction
   https://docs.pingcap.com/tidb/stable/partitioned-table#reorganize-partitions
   https://docs.pingcap.com/tidb/stable/partitioned-table#manage-hash-and-key-partitions
   https://docs.pingcap.com/tidb/stable/partitioned-table#convert-a-partitioned-table-to-a-non-partitioned-table
   https://docs.pingcap.com/tidb/stable/partitioned-table#partition-an-existing-table
   https://docs.pingcap.com/tidb/dev/partitioned-table#global-indexes

[Oreoxmt]

@D3Hunter Could you please take a look at this issue?