Update docs

Author: danez

docs/reference.md

@@ -718,10 +718,25 @@ Parses string containing GraphQL query or [type definition](type-system/type-lan
  * in the source that they correspond to. This configuration flag
  * disables that behavior for performance or testing.)
  *
+ * experimentalFragmentVariables: boolean,
+ * (If enabled, the parser will understand and parse variable definitions
+ * contained in a fragment definition. They'll be represented in the
+ * `variableDefinitions` field of the FragmentDefinitionNode.
+ *
+ * The syntax is identical to normal, query-defined variables. For example:
+ *
+ *   fragment A($var: Boolean = false) on T  {
+ *     ...
+ *   }
+ *
+ * Note: this feature is experimental and may change or be removed in the
+ * future.)
+ *
  * @api
  * @param Source|string $source
  * @param array $options
  * @return DocumentNode
+ * @throws SyntaxError
  */
 static function parse($source, array $options = [])
 ```