Updating extension guidelines v2

Continuing #1035 and #1218, #1135 related.
Feel free to just drop my commit on this. I am just plain confused on the recommended way for a user to import an extension--flask_foo vs flask.ext.foo... 

The whole reason I made this commit was because the docs seemed to conflict with what you wrote in https://github.com/mitsuhiko/flask/issues/1092#issuecomment-47118613 , as well as https://github.com/mitsuhiko/flask/pull/1085#issuecomment-45466907 so I thought I'd simply fix the docs. 

But after reading through your reply at https://github.com/mitsuhiko/flask/pull/1220#issuecomment-60509578 , I'm a bit more confused than I was before, so I'll just leave clarifying the docs to someone who knows more than I do.