Add missing FeatureHolder methods to FML spec

[jaredlockhart]

The FML specification (docs/technical-reference/fml/fml-spec.mdx) only documents value() and recordExposure() from the FeatureHolder API. Several methods available in the source code are undocumented:

Missing methods:

• recordExperimentExposure(slug) — exposure recording for co-enrolling features
• recordMalformedConfiguration(partId) — report invalid experiment configurations as Glean telemetry
• toJSONObject() — get raw JSON representation of feature configuration
• Testing helpers: withCachedValue(), withInitializer(), withSdk(), isUnderTest()