update async commit related configurations (#4537)

[ti-srebot]

cherry-pick #4537 to release-5.0

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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)

Async commit and 1PC configurations have been moved from config file to system variables.

And there are known correctness issues about 1PC, so 1PC content is hidden.

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: update async commit related configurations docs-cn#5181
• 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-srebot]

@Joyinqin you're already a collaborator in bot's repo.