Docs: Improve documentation for format_for_editor() and the 'the_editor_content' filter it is hooked to.

Props AramZS for initial patch.
Fixes #34866.

git-svn-id: https://develop.svn.wordpress.org/trunk@35904 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-editor.php

@@ -261,7 +261,9 @@ final class _WP_Editors {
 		 *
 		 * @since 2.1.0
 		 *
-		 * @param string $content Default editor content.
+		 * @param string $content        Default editor content.
+		 * @param string $default_editor The default editor for the current user.
+		 *                               Either 'html' or 'tinymce'.
 		 */
 		$content = apply_filters( 'the_editor_content', $content, $default_editor );