docs(errors/$compile/nonassing): update nonassign.ngdoc

It might seem obvious that if you don't supply "bind" attribute in this case, you'll get an error,
but I feel this is worth adding to the doc.

Closes #6725

docs/content/error/$compile/nonassign.ngdoc

@@ -30,6 +30,9 @@ Following are invalid uses of this directive:
 
 <!-- ERROR because `myFn()=localValue` is an invalid statement -->
 <my-directive bind="myFn()">
+
+<!-- ERROR because attribute bind wasn't provided -->
+<my-directive>
 ```