MimeMapMediaHandler.php: add minor doc

inc/Service/Media/MimeMapMediaHandler.php

@@ -29,6 +29,10 @@ class MimeMapMediaHandler implements MediaHandler {
 		);
 	}
 
+	/**
+	 * @var callable(string): ?string Maps a mime type to a path to an image to be used as thumbnail, or null if the
+	 *                                mime is not supported.
+	 */
 	public function __construct(callable $mime_mapper) {
 		$this->mime_mapper = $mime_mapper;
 	}