Audit and update API/code documentation for filter field/prefixing changes (PR #1291)

[sscargal]

For the filter prefixing and validation changes in PR #1291, audit all current API and code documentation for sufficient explanation of:

• The required m./metadata. prefix in all user metadata filter fields
• The explicit 400 error behavior for unknown/misspelled fields

If documentation is unclear or incomplete in any location (README, docs site, or API usage documentation), update those docs to match the new syntax and behavior.

If required, open issues for locations where extra explanation/examples are needed.

Reference: See OpenAPI update issue and PR #1291.