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/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/ImageLoaderHolder.kt

@@ -11,8 +11,24 @@ import coil3.ImageLoader
 import io.element.android.libraries.matrix.api.MatrixClient
 import io.element.android.libraries.matrix.api.core.SessionId
 
+/**
+ * Caches one Coil image loader per session, since loading Matrix media requires the session's own client to fetch and decrypt it.
+ */
 interface ImageLoaderHolder {
+    /** Returns a loader that can only fetch ordinary URLs, for images that do not come from a Matrix room. */
     fun get(): ImageLoader
+
+    /**
+     * Returns the loader of a session, creating it on first use.
+     *
+     * @param client the session whose media the loader will fetch.
+     */
     fun get(client: MatrixClient): ImageLoader
+
+    /**
+     * Drops the cached loader of a session, to be called when that session goes away.
+     *
+     * @param sessionId the session whose loader is dropped.
+     */
     fun remove(sessionId: SessionId)
 }

libraries/matrixmedia/api/src/main/kotlin/io/element/android/libraries/matrix/ui/media/InitialsAvatarBitmapGenerator.kt

@@ -14,6 +14,15 @@ import io.element.android.libraries.designsystem.components.avatar.AvatarData
  * Generates a bitmap for an initials avatar based on the provided [io.element.android.libraries.designsystem.components.avatar.AvatarData].
  */
 interface InitialsAvatarBitmapGenerator {
+    /**
+     * Draws the initials avatar, used where a bitmap is required rather than a Composable, such as in notifications and shortcuts.
+     *
+     * @param size the width and height of the bitmap in pixels.
+     * @param avatarData the name and id the initials and the background colour are derived from.
+     * @param useDarkTheme whether to draw the dark theme variant.
+     * @param fontSizePercentage the text height as a fraction of [size].
+     * @return the bitmap, or `null` when it could not be drawn.
+     */
     fun generateBitmap(
         size: Int,
         avatarData: AvatarData,