config, sysvar: update descriptions about instance sysvars (#10978)

[ti-chi-bot]

This is an automated cherry-pick of #10978

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).

• 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):
  • sysvar, config: mapping config settings to system variables. tidb#33279
  • *: move config file option oom-use-tmp-storage to sysvar tidb#36679
  • config, sysvar: add config instance.enable_ddl and sysvar tidb_enable_ddl tidb#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]

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[TomShawn]

/hold

[ti-chi-bot]

@ti-chi-bot: PR needs rebase.

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[ti-chi-bot]

@ti-chi-bot: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	593d552	link	true	/test pull-verify

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