system-variables: Add units wherever possible

[shichun-0415]

What is changed, added or deleted? (Required)

This adds units wherever possible, and removes it from the body description if redundant.

This helps improve readability and slightly reduce risk of misconfiguration.

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)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.4 (TiDB 6.4 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: system-variables: Add units wherever possible docs#6622
• 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:

• 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.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[TomShawn]

LGTM

[shichun-0415]

/merge

[ti-chi-bot]

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

Commit hash: 0a499b3

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #12915.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.3: #12916.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.4: #12917.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #12918.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.4: #12919.