Add feature flags user lists endpoints to OpenApi v2 docs

What does this MR do and why?

This merge request adds endpoints from API::FeatureFlagsUserLists to the OpenApi V2 documentation as per #211512.

Please note that:

• Changes to doc/api/openapi/openapi_v2.yaml were excluded to avoid conflicts within the same documentation effort.
• Errors related to consumes property were ignored as per https://gitlab.com/gitlab-org/gitlab/-/issues/377941.
• Descriptions of all endpoints were updated to match the markdown documentation.
• We use multiple types for params that accept them (even if it's not supported in OpenApi V2) as per !101875 (comment 1148561131).

More guidelines on these changes can be found in &8926 (comment 1151742900).

Resolves #376906.

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