Docs: Describe the $feedname argument of add_feed().

Fixes #34259.

git-svn-id: https://develop.svn.wordpress.org/trunk@35119 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rewrite-functions.php

@@ -92,7 +92,7 @@ function add_permastruct( $name, $struct, $args = array() ) {
  *
  * @global WP_Rewrite $wp_rewrite
  *
- * @param string   $feedname
+ * @param string   $feedname Feed name.
  * @param callable $function Callback to run on feed display.
  * @return string Feed action name.
  */