docs: fix expression index's document again (#6295)

[ti-chi-bot]

This is an automated cherry-pick of #6295

What is changed, added or deleted? (Required)

revised descriptions about expression indexes in "sql-statements/sql-statement-create-index.md", "system-variables.md", and "tidb-configuration-file.md".

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• v2.1 (TiDB 2.1 versions)

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

• This PR is translated from: fix expression index's document again. docs-cn#6950 add a comment for expression index docs-cn#6995
• 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]

[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: dc999ea