improve comments on settings::update() method

lib/plugins/config/settings/config.class.php

@@ -388,10 +388,12 @@ if (!class_exists('setting')) {
     }
 
     /**
-     *  update setting with user provided value $input
-     *  if value fails error check, save it
+     *  update changed setting with user provided value $input
+     *  - if changed value fails error check, save it to $this->_input (to allow echoing later)
+     *  - if changed value passes error check, set $this->_local to the new value
      *
-     *  @return boolean true if changed, false otherwise (incl. on error)
+     *  @param  mixed   $input   the new value
+     *  @return boolean          true if changed, false otherwise (incl. on error)
      */
     function update($input) {
         if (is_null($input)) return false;