Added COALESCE PARTITION

[mjonss]

What is changed, added or deleted? (Required)

Added documentation for COALESCE PARTITION and ADD PARTITION for Hash/Key partitioned tables.

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

• This PR is translated from:
• Other reference link(s):
  ddl: Coalesce partition | tidb-test=pr/2111 tidb#42476

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[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• qiancai
• ran-huang

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[qiancai]

Do we need to update the statememt to SHOW CREATE TABLE example;

[mjonss]

I think it is still correct.

[bb7133]

LGTM

[ran-huang]

Rest LGTM

[qiancai]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: d484da8