Add doc for tidb_executor_concurrency (#4602)

[ti-srebot]

cherry-pick #4602 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/4603

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

git push git@github.com:ti-srebot/docs.git pr/4603:release-5.0-1969f0d31b10

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What is changed, added or deleted? (Required)

Add docs for the new system variable, tidb_executor_concurrency.

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: Add doc for tidb_executor_concurrency docs-cn#3975
• 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]

@TomShawn please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs/invitations