Document default successCallback, errorCallback, notfiyCallback for promise.then(...)

[fiveop]

The documentation for the then method of promises constructed using the $q service does not describe the behaviour of the default callbacks.

After I figure out what the defaults are, I will draft a patch.

[btford]

Perhaps the docs for Angular's promises could be clearer, but I don't think "default callbacks" clarifies. I think it might be better to note that the error and notification callbacks are optional in .then.