Adding documentation to repeat_optional.js

src/js/parser/javascript/repeat_optional.js

@@ -1,3 +1,7 @@
+// RepeatOptional nodes are used for `a?` regular expression syntax. It is not
+// rendered directly; it just indicates that the [Repeat](./repeat.html) node
+// loops zero or one times.
+
 export default {
   minimum: 0,
   maximum: 1