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/contentscanner/api/src/main/kotlin/io/element/android/features/contentscanner/api/ContentScannerService.kt

@@ -18,5 +18,11 @@ import io.element.android.libraries.matrix.ui.media.contentvalidation.ContentVal
  * This will only process media when a [ContentScanner] is provided when building the client in Pro.
  */
 fun interface ContentScannerService {
+    /**
+     * Starts scanning the given media, reporting the verdict of each one through [contentValidationState] rather than by returning.
+     *
+     * @param mediaSources the media to scan.
+     * @param contentValidationState where the outcome of each scan is published for the UI to observe.
+     */
     fun scan(mediaSources: List<MediaSource>, contentValidationState: ContentValidationState)
 }