tidb: add explanation for the fields in mysql.user

[CbcWestwolf]

What is changed, added or deleted? (Required)

• add explanation for the fields in mysql.user
• put mysql-schema*.md in a unique dir

close #16684

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[dveeden]

1. It might be good to put a reminder on this page that these are system tables and that it is not recommended to change them directly. The recommended way is to use CREATE USER/ALTER USER/DROP USER/GRANT/REVOKE/SHOW CREATE USER etc.
2. When people still want to make changes to these tables directly they should run FLUSH PRIVILEGES.
3. Do we want/need to put in a statement about what is MySQL compatible and what's not?
4. Would still be good to have a page per table instead of one page for all tables combined. Just like information_schema descriptions.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-22 06:11:21.607350432 +0000 UTC m=+852703.598291901: ☑️ agreed by dveeden.
• 2024-07-25 06:37:18.224016212 +0000 UTC m=+1113460.214957681: ☑️ agreed by Oreoxmt.

[lilin90]

/lgtm

[Oreoxmt]

/approve

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #18417.