config, sysvar: update descriptions about instance sysvars

[CbcWestwolf]

First-time contributors' checklist

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

What is changed, added or deleted? (Required)

Change descriptions about instance system variables and [instance] config items.

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).

• [x] master (the latest development version)
• [ ] v6.2 (TiDB 6.2 versions)
• [ ] v6.1 (TiDB 6.1 versions)
• [ ] v5.4 (TiDB 5.4 versions)
• [ ] v5.3 (TiDB 5.3 versions)
• [ ] v5.2 (TiDB 5.2 versions)
• [ ] v5.1 (TiDB 5.1 versions)
• [ ] v5.0 (TiDB 5.0 versions)

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

• This PR is translated from:
• Other reference link(s):
  • https://github.com/pingcap/tidb/pull/33279
  • https://github.com/pingcap/tidb/pull/36679
  • https://github.com/pingcap/tidb/pull/35425

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:

• TomShawn

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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[TomShawn]

/remove-status LGT1 /status LGT2

[CbcWestwolf]

/run-verify

[CbcWestwolf]

/run-test

[TomShawn]

/verify

[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 50d3e43e7054371280a4c7c549607a5be1ad9d29

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11840.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11841.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #11842.