[4.4] Add missing @return annotations

Author: nicolas-grekas

Form.php

@@ -346,6 +346,8 @@ public function offsetGet($name)
      * @param string       $name  The field name
      * @param string|array $value The value of the field
      *
+     * @return void
+     *
      * @throws \InvalidArgumentException if the field does not exist
      */
     public function offsetSet($name, $value)
@@ -357,6 +359,8 @@ public function offsetSet($name, $value)
      * Removes a field from the form.
      *
      * @param string $name The field name
+     *
+     * @return void
      */
     public function offsetUnset($name)
     {