Add KDoc to the single-interface library and service API modules

Cover the last remaining interfaces across libraries/audio, libraries/cachestore,
libraries/indicator, libraries/slashcommands, libraries/voiceplayer and
services/apperror, with their members and parameters.

This completes the effort: all 222 in-scope interfaces and their 820 members in
the 60 api modules that hold any now carry documentation.

Notable points: CacheStore applies no expiry of its own and only records when each
entry was written, leaving staleness to the caller; SlashCommandService.parse
never throws, returning NotACommand for ordinary text and an error case for a
malformed command.

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

libraries/indicator/api/src/main/kotlin/io/element/android/libraries/indicator/api/IndicatorService.kt

@@ -15,9 +15,11 @@ import androidx.compose.runtime.State
  * A set of State<Boolean> to observe to display or not the indicators in the UI.
  */
 interface IndicatorService {
+    /** Whether to show the red dot on the room list top bar, which aggregates the reasons the user should open the settings. */
     @Composable
     fun showRoomListTopBarIndicator(): State<Boolean>
 
+    /** Whether to show the red dot on the chat backup setting, i.e. when recovery needs the user's attention. */
     @Composable
     fun showSettingChatBackupIndicator(): State<Boolean>
 }