Optional feature documentation should display expected values per Edition

[locks]

Since the values of each optional feature depend on which edition the application is on, we should make sure the documentation displays such information.

[rwjblue]

It isn't exactly clear how best to display this info. Specifically, we basically support any specific value for the optional features on any version of Ember / Ember CLI that support the version.

I think showing the version that introduced the optional feature (this is the first version that allows the flag to be used), and the current default value (default meaning the value used in the output of ember new foo) is probably the best we can do.

@locks What do you think?