Add KDoc to six small library API modules

Cover the remaining interfaces of libraries/troubleshoot, libraries/matrixmedia,
libraries/dateformatter, libraries/featureflag, libraries/workmanager and
libraries/mediaviewer, along with their members and parameters.

Behaviours worth recording: NotificationTroubleshootTest reports its outcome
through its state flow rather than by returning, and its quickFix throws unless
the test offers one; DateFormatter.format turns a null timestamp into an empty
string so callers need no null check; ImageLoaderHolder keeps one loader per
session because decrypting room media needs that session's client.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/Feature.kt

@@ -10,6 +10,9 @@ package io.element.android.libraries.featureflag.api
 
 import io.element.android.libraries.core.meta.BuildMeta
 
+/**
+ * Describes one feature flag: how it is identified, how it is presented in the developer options, and what it defaults to.
+ */
 interface Feature {
     /**
      * Unique key to identify the feature.

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlagService.kt

@@ -11,6 +11,9 @@ package io.element.android.libraries.featureflag.api
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.first
 
+/**
+ * Reads and, where a mutable provider is registered, changes the state of the feature flags.
+ */
 interface FeatureFlagService {
     /**
      * @param feature the feature to check for