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/slashcommands/api/src/main/kotlin/io/element/android/libraries/slashcommands/api/SlashCommandService.kt

@@ -9,7 +9,19 @@ package io.element.android.libraries.slashcommands.api
 
 import io.element.android.libraries.matrix.api.timeline.Timeline
 
+/**
+ * Recognises and runs the slash commands the user can type in the composer, such as `/me` or `/join`.
+ *
+ * The composer first [parse]s what was typed, then hands the result to whichever proceed method matches its kind; navigation
+ * commands and the error cases are handled by the composer itself.
+ */
 interface SlashCommandService {
+    /**
+     * Returns the commands whose name starts with what has been typed, so the composer can offer an autocomplete list.
+     *
+     * @param text the current composer content.
+     * @param isInThread whether the composer is in a thread, since not every command is available there.
+     */
     suspend fun getSuggestions(
         text: String,
         isInThread: Boolean,
@@ -17,6 +29,11 @@ interface SlashCommandService {
 
     /**
      * Parse the message and return a SlashCommand.
+     * Ordinary text comes back as `NotACommand`, and a malformed or unsupported command as one of the error cases, so this never throws.
+     *
+     * @param textMessage the composer content as plain text.
+     * @param formattedMessage the composer content as HTML, or `null` when unformatted.
+     * @param isInThreadTimeline whether the composer is in a thread.
      */
     suspend fun parse(
         textMessage: CharSequence,
@@ -26,6 +43,9 @@ interface SlashCommandService {
 
     /**
      * Proceed a SlashCommandSendMessage.
+     *
+     * @param slashCommand the parsed command, carrying the content to send.
+     * @param timeline the timeline to send it to.
      */
     suspend fun proceedSendMessage(
         slashCommand: SlashCommand.SlashCommandSendMessage,
@@ -33,7 +53,9 @@ interface SlashCommandService {
     ): Result<Unit>
 
     /**
-     * Proceed a SlashCommandAdmin.
+     * Proceed a SlashCommandAdmin, i.e. one that acts on the room or its members rather than sending a message.
+     *
+     * @param slashCommand the parsed command to run.
      */
     suspend fun proceedAdmin(
         slashCommand: SlashCommand.SlashCommandAdmin,