docs(ngController): add more description of `controller as` syntax

Using `controller as` in the template is not described well
in the docs, as both `scope` injection and `this` are presented
equally without too much discussion of the advantages of using
either. I added a bit more discussion based on google's internal
style guidelines.

Closes #7591
Closes #5076 (until Angular 2.0 comes out and we refactor everything)