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, because this was my error, and the documentation led me to try every permutation of attribute values on <my-directive>1, and completely missed the fact that <my-directive>2 in my code didn't have ANY attribute.  If my proposed edit had been there, I might have found my problem earlier.