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>

services/apperror/api/src/main/kotlin/io/element/android/services/apperror/api/AppErrorStateService.kt

@@ -11,10 +11,26 @@ package io.element.android.services.apperror.api
 import androidx.annotation.StringRes
 import kotlinx.coroutines.flow.StateFlow
 
+/**
+ * Surfaces app-wide error dialogs from anywhere, for the failures that are not tied to the screen the user is on.
+ */
 interface AppErrorStateService {
+    /** The error to display, if any; observed once at the top of the app rather than per screen. */
     val appErrorStateFlow: StateFlow<AppErrorState>
 
+    /**
+     * Shows an error dialog with already resolved text.
+     *
+     * @param title the dialog title.
+     * @param body the dialog message; never put user content or secrets in it.
+     */
     fun showError(title: String, body: String)
 
+    /**
+     * Shows an error dialog from string resources, which is the preferred form since it stays localised.
+     *
+     * @param titleRes resource id of the dialog title.
+     * @param bodyRes resource id of the dialog message.
+     */
     fun showError(@StringRes titleRes: Int, @StringRes bodyRes: Int)
 }