docs(tutorial/step_05): explain need for $httpBackend.flush in tests

There is an excellent explanation for the need for this in the documentation that may be helpful to
tutorial users, so I added a link to it.

Closes #9919
docs(tutorial/step_12): small change to overview

The bullet points at the beginning of the article were a little hard to understand because they
didn't follow the grammatical form of the preceding articles. I hope these small modifications make
it a little easier for someone else to read.

Closes #9922