Improve documentation around 'concrete' in bindings. No bug. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D35465

--HG--
extra : moz-landing-system : lando

dom/bindings/Bindings.conf

@@ -21,7 +21,8 @@
 #   * concrete - Indicates whether there exist JS objects with this interface as
 #                their primary interface (and hence whose prototype is this
 #                interface's prototype object).  Always False for callback
-#                interfaces.  Defaults to True otherwise.
+#                interfaces.  Defaults to True for leaf interfaces and
+#                interfaces with constructors, false otherwise.
 #   * notflattened - The native type does not have nsIClassInfo, so when
 #                    wrapping it the right IID needs to be passed in.
 #                    Only relevant for callback interfaces.