tidb: add explanation for the fields in mysql.user (#18118)

[ti-chi-bot]

This is an automated cherry-pick of #18118

What is changed, added or deleted? (Required)

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

Which TiDB version(s) do your changes apply to? (Required)

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• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
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• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)

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

• This PR is translated from: tidb: add explanation for the fields in mysql.user docs#18318
• 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

[Oreoxmt]

/approve

[ti-chi-bot]

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