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/cachestore/api/src/main/kotlin/io/element/android/libraries/cachestore/api/CacheStore.kt

@@ -7,9 +7,33 @@
 
 package io.element.android.libraries.cachestore.api
 
+/**
+ * A generic on-disk key/value cache, where each entry records when it was written so callers can decide for themselves when it is stale.
+ */
 interface CacheStore {
+    /**
+     * Stores an entry, replacing any previous one under the same key.
+     *
+     * @param key the entry to write.
+     * @param data the value along with the time it was obtained.
+     */
     suspend fun storeData(key: String, data: CacheData)
+
+    /**
+     * Reads an entry without applying any expiry of its own; judging staleness is up to the caller.
+     *
+     * @param key the entry to read.
+     * @return the value and its timestamp, or `null` when nothing is stored under that key.
+     */
     suspend fun getData(key: String): CacheData?
+
+    /**
+     * Removes one entry.
+     *
+     * @param key the entry to delete.
+     */
     suspend fun deleteData(key: String)
+
+    /** Empties the whole cache. */
     suspend fun deleteAll()
 }