Update & clarify docs for pubspec.yaml documentation field #801
Description
In the course of researching angulardart/angular_components#252, I looked at the docs for pubspec's documentation field. They need work.
They seem to imply that you won't get pub-generated API docs if you don't specify a blank documentation field. That can't be true. I looked at the file history, and this text has been there for at least 2 years, long before pub started hosting API docs.
@isoos, can you give info here about how the documentation field is treated?