tidb: add explanation for the fields in mysql.user (#18318)

[ti-chi-bot]

This is an automated cherry-pick of #18318

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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Do your changes match any of the following descriptions?

• Delete files
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• Might cause conflicts after applied to another branch

[Oreoxmt]

/approve

[ti-chi-bot]

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