Make inlineCallbacks more proeminent in the docs

[Leonardo Santagada]

inlineCallbacks is more or less what has been reinvented many times in the python community and I think it is partially because it is buried in the twisted documentation.

There should be a way to show inlineCallbacks to new users to show that you can do asynchronous communication without using callbacks everywhere. In the last pycon(2011) guido himself told that he doesn't understand callback based code and that he thinks the future of async in python is using generators, so I really think this is something the twisted community should talk about.

[Jean-Paul Calderone]

This seems like a duplicate of #1009 to me.