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/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/WorkManagerScheduler.kt

@@ -10,20 +10,31 @@ package io.element.android.libraries.workmanager.api
 
 import io.element.android.libraries.matrix.api.core.SessionId
 
+/**
+ * Schedules background work, wrapping `WorkManager` so the rest of the app does not depend on it directly.
+ */
 interface WorkManagerScheduler {
     /**
      * Submits a new work request built from [workManagerRequestBuilder] to run in `WorkManager`.
+     *
+     * @param workManagerRequestBuilder describes the work to schedule.
      */
     suspend fun submit(workManagerRequestBuilder: WorkManagerRequestBuilder)
 
     /**
      * Checks if there are any pending requests scheduled for the provided [sessionId] and [requestType].
+     *
+     * @param sessionId the session the work belongs to.
+     * @param requestType the kind of work to look for.
      */
     fun hasPendingWork(sessionId: SessionId, requestType: WorkManagerRequestType): Boolean
 
     /**
      * Cancel pending work requests for the session [SessionId].
      * If [requestType] is provided, it will only cancel requests for that type, otherwise it will cancel all requests.
+     *
+     * @param sessionId the session whose work is cancelled.
+     * @param requestType the kind of work to cancel, or `null` to cancel every kind.
      */
     fun cancel(sessionId: SessionId, requestType: WorkManagerRequestType? = null)
 }

libraries/workmanager/api/src/main/kotlin/io/element/android/libraries/workmanager/api/di/MetroWorkerFactory.kt

@@ -28,7 +28,13 @@ class MetroWorkerFactory(
       return workerProviders[Class.forName(workerClassName).kotlin]?.create(workerParameters)
     }
 
+    /**
+     * Creates one worker type with its dependencies injected, since `WorkManager` instantiates workers itself and cannot use the graph.
+     */
     interface WorkerInstanceFactory<T : ListenableWorker> {
+      /**
+       * @param params the parameters `WorkManager` built the worker with.
+       */
       fun create(params: WorkerParameters): T
     }
 }