partitioning: Corrected partition management

[mjonss]

Clarified currently supported partitioning management, and added experimental feature EXCHANGE PARTITION.
Fixes pingcap#5424

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What is changed, added or deleted? (Required)

Clarified currently supported partitioning management, and added experimental feature EXCHANGE PARTITION.
Fixes pingcap#5424
It mentioned split, merge and redefine, which is not clear what operations it refers to (REORGANIZE PARTITION, which is not supported)

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): "split, redefine partitions by using ALTER TABLE statements" is incorrect pingcap/docs#5424

Do your changes match any of the following descriptions?

• Delete files
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• Need modification after applied to another branch
• Might cause conflicts after applied to another branch