Show the unstable params and improve collection docs

[obulat]

Fixes

Fixes #4080 by @obulat
Fixes #4081 by @obulat

Description

This PR makes all unstable__ search parameters visible in the documentation and adds the warning about them being unstable.
It also updates the collection parameters docs: uses a variable for parameter name to ensure that currently we only use unstable__collection and unstable__tag in the documentation.

Testing Instructions

Run the API and read the search endpoint documentation. All unstable fields should be displayed and have a warning. The collection params should have unstable__ prefix.

Checklist

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[stacimc]

LGTM!

[openverse-bot]

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