docs(guide/Directives): demonstrate how to pass data from isolate to parent scope

It looks like this used to be in the Angular docs as per this thread:
https://groups.google.com/d/msg/angular/3CHdR_THaNw/AxqKwUw5t0oJ. I recently
spent some time trying to get this to work and was very frustrated by lack of
documentation.

Closes #10567