Update the related variable description for CREATE INDEX

[qiancai]

What is changed, added or deleted? (Required)

Update the related variable description for CREATE INDEX because the scope of tidb_ddl_reorg_priority is SESSION instead of GLOBAL.

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

• master (the latest development version)
• v5.1 (TiDB 5.1 versions)
• 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): https://asktug.com/t/topic/94169

Do your changes match any of the following descriptions?

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

[qiancai]

/assign

[TomShawn]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn
• tangenta

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[qiancai]

/merge

[ti-chi-bot]

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

Commit hash: 5233c54

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6605.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6606.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6607.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6608.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #6609.