Update severity/threshold section and add nocheck section

[TylerJang27]

• Update information to the severity/threshold section to note that we also support the note level (and update output types accordingly).
• Document that we surface info on unused diagnostics, and that these can be ignored with trunk-ignore(trunk)
• Run trunk upgrade

Note: this requires 1.22.1 or later with the note severity level and the nocheck fixes.

[trunk-io[bot]]

⏱️ 16s total CI duration on this PR

Job	Cumulative Duration	Recent Runs
build	16s	🟩 🟩

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