Add KDoc to the single-interface feature API modules

Cover the last remaining interfaces across features/announcement, cachecleaner,
call, contentscanner, createroom, knockrequests, lockscreen, rolesandpermissions,
startchat, userprofile and viewfolder, with their members and parameters.

Grouped into one commit rather than eleven near-empty ones, per the AGENTS.md rule
about tiny commits.

This completes every features/* api module.

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

features/cachecleaner/api/src/main/kotlin/io/element/android/features/cachecleaner/api/CacheCleaner.kt

@@ -8,6 +8,9 @@
 
 package io.element.android.features.cachecleaner.api
 
+/**
+ * Removes the temporary files holding decrypted content, so plaintext media does not linger on disk.
+ */
 interface CacheCleaner {
     /**
      * Clear the cache subdirs holding temporarily decrypted content (such as media and voice messages).