Document that --uri should be a full URL

[weitzman]

Now that D8 does not support setting base_url in settings.php, its imperative that folks pass --uri=http://example.com using config or site alias. Otherwise, commands like browse and user-login will return bad urls. ping to @greg-1-anderson who may know what docs need to be updated.

[greg-1-anderson]

Touched up the documentation pages that mention uri.