Update docs for tidb_allow_function_for_expression_index

[Oreoxmt]

What is changed, added or deleted? (Required)

1. The default value of tidb_allow_function_for_expression_index was incorrect as it was formatted not a single string, but as a set of strings.
2. Link from the variable to the expression index doc
3. Link to the functions from the expression index doc

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.

• [x] master (the latest development version)
• [x] v8.2 (TiDB 8.2 versions)
• [x] v8.1 (TiDB 8.1 versions)
• [ ] v8.0 (TiDB 8.0 versions)
• [ ] v7.6 (TiDB 7.6 versions)
• [ ] v7.5 (TiDB 7.5 versions)
• [ ] v7.1 (TiDB 7.1 versions)
• [ ] v6.5 (TiDB 6.5 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)

What is the related PR or file link(s)?

• This PR is translated from: https://github.com/pingcap/docs/pull/17690
• 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[bot]]

[LGTM Timeline notifier]

Timeline:

• 2024-06-26 05:47:01.828717475 +0000 UTC m=+784948.314206307: :ballot_box_with_check: agreed by qiancai.

[Oreoxmt]

/approve

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Oreoxmt

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #17795.