"split, redefine partitions by using ALTER TABLE statements" is incorrect

[kolbe]

https://docs.pingcap.com/tidb/stable/partitioned-table#range-partition-management makes this statement:

You can add, drop, merge, split, redefine partitions by using ALTER TABLE statements.

We do not currently support "split" or "redefine" partition operations, as far as I know and can tell by my testing.

File: /partitioned-table.md

[zz-jason]

@qw4990 we need to verify and correct the document related to partition table

[mjonss]

/cc @mjonss

[TomShawn]

/assign @mjonss