docs: clarify TiUP mirror clone for partial upgrades

[xhebox]

What is changed, added or deleted? (Required)

• Clarifies that tiup mirror clone --pd, --tikv, and similar component filters only clone the selected component packages and do not automatically clone the cluster component.
• Adds an offline partial component upgrade example that explicitly clones both the target component and the cluster component.
• Explains why the keys directory needs to be copied before merging mirrors.

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

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

• This PR is translated from:
• 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

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