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/audio/api/src/main/kotlin/io/element/android/libraries/audio/api/AudioFocus.kt

@@ -15,6 +15,9 @@ enum class AudioFocusRequester {
     MediaViewer,
 }
 
+/**
+ * Claims and releases system audio focus, so that the app's own players do not talk over each other or over other apps.
+ */
 interface AudioFocus {
     /**
      * Request audio focus for the given requester.