update enable-ttl description

[7yyo]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

Update enable-ttl description

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• 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):

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• en-jin19
• hfxsd

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

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[hfxsd]

LGTM

[en-jin19]

LGTM

[en-jin19]

/merge

[ti-chi-bot]

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

Commit hash: c48dcda

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #8579.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #8580.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #8581.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #8582.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #8583.