docs: document metaMetricsIds explicit targeting for threshold flags

[pedronfigueiredo]

Summary

• Adds a new #### Explicit MetaMetrics ID targeting subsection to docs/remote-feature-flags.md
• Documents the metaMetricsIds field on threshold entries: purpose, precedence rules, matching behaviour, and a combined explicit-ID + percentage-rollout example
• Includes a callout that this is for QA/PM testing only, not general segmentation

Test plan

• Docs render correctly in preview

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Note

Low Risk
Documentation-only change with no runtime or configuration code modified.

Overview
Adds a new Explicit MetaMetrics ID targeting section to docs/remote-feature-flags.md, describing the optional metaMetricsIds array on threshold flag entries.

The doc explains that QA/PM can pin specific users to a threshold entry before hash-based rollout runs, with precedence (local overrides → explicit ID match → hash), matching rules (case-insensitive trim, first entry wins, malformed arrays skipped), redaction of IDs in logs/state, and a worked example mixing pinned IDs with a 30%/70% rollout. It also notes compatibility with legacy threshold entries and that explicit matches do not use the threshold cache.

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