system variable: add tidb_allow_fallback_to_tikv (#4781)

[ti-srebot]

cherry-pick #4781 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/5103

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

git push git@github.com:ti-srebot/docs.git pr/5103:release-5.0-0d2f8987963f

---

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)

Add tidb_allow_fallback_to_tikv into system variables.

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: system variable: add tidb_allow_fallback_to_tikv docs-cn#5484
• Other reference link(s): server: retry executing sql without tiflash after tiflash is down tidb#22459, *: rename tiflash fallback switch tidb#22886

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]

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

[TomShawn]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 71ceb27