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/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DateFormatter.kt

@@ -8,7 +8,17 @@
 
 package io.element.android.libraries.dateformatter.api
 
+/**
+ * Formats timestamps for display, in the user's locale and time zone.
+ */
 interface DateFormatter {
+    /**
+     * Formats a timestamp according to [mode].
+     *
+     * @param timestamp milliseconds since the epoch; a `null` value yields an empty string, so callers need no null check of their own.
+     * @param mode how much of the date and time to include; see [DateFormatterMode] for examples of each.
+     * @param useRelative true to prefer wording relative to today, such as "Today" or "This month", where the mode allows it.
+     */
     fun format(
         timestamp: Long?,
         mode: DateFormatterMode = DateFormatterMode.Full,

libraries/dateformatter/api/src/main/kotlin/io/element/android/libraries/dateformatter/api/DurationFormatter.kt

@@ -21,6 +21,11 @@ import kotlin.time.Duration
  * - 30 seconds → "30 seconds"
  */
 interface DurationFormatter {
+    /**
+     * Formats the duration using its largest appropriate unit, rounding rather than listing several units.
+     *
+     * @param duration the duration to describe.
+     */
     fun format(duration: Duration): String
 }