improve system-variables documentation (#4598)

[ti-srebot]

cherry-pick #4598 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In docs repo:
git pr https://github.com/pingcap/docs/pull/4647

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/docs.git pr/4647:release-5.0-3339a09457ab

---

What is changed, added or deleted? (Required)

The system variables documentation is incomplete and out of date. This helps improve it, but there are still many issues.

The following issues were fixed:

• new system variables are not documented in alphabetical order #4532 variables did not appear in alphabetical order
• default values for booleans are incorrect #4531 Default values for booleans are incorrect
• TiDB executor concurrency is not documented (this was introduced in master and is an important setting. If Add doc for tidb_executor_concurrency docs-cn#3975 merges, the translation can be overwritten here.)
• The defaults for other executor concurrency options were incorrect. They also did not document if they were deprecated by executor concurrency.
• The new options added have backticks in the heading. I've removed them for consistency.
• The options auto_increment_increment, auto_increment_offset, port, socket, system_time_zone are now documented.
• The description of tidb_check_mb4_value_in_utf8 was vague/incorrect. It checks for data outside of the BMP (i.e. "mysql utf8" not UTF-8).

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:
• 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]

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

[TomShawn]

LGTM