docs: modify settings doc

[TCeason]

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Summary

modify docs about settings

Fixes part of #7318

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Name	Status	Preview	Updated
databend	✅ Ready (Inspect)	Visit Preview	Sep 2, 2022 at 4:15AM (UTC)

[mergify]

This pull request's title is not fulfill the requirements. @TCeason please update it 🙏.

Valid format:

fix(query): fix group by string bug
  ^         ^---------------------^
  |         |
  |         +-> Summary in present tense.
  |
  +-------> Type: rfc, feat, fix, refactor, ci, docs, chore

Valid types:

• rfc: this PR proposes a new RFC
• feat: this PR introduces a new feature to the codebase
• fix: this PR patches a bug in codebase
• refactor: this PR changes the code base without new features or bugfix
• ci: this PR changes build/testing/ci steps
• docs: this PR changes the documents or websites
• chore: this PR only has small changes that no need to record

[TCeason]

Maybe we need add a new folder: docs/doc/30-reference/40-identifier.

Used to describe ident