Documentation improovenemt for auxiliary methods

[fruboes]

Hi,

by a total chance, I've run into this fragment on main page (http://spyne.io/#auxproc=Sync&s=aux :)

To do additional work after the primary method returns, you can set the aux attribute in another service definition class and implement the same method signature

For a total beginner in spyne/rpc world (that is me) this was quite enlighting in terms of what is happening. Earlier, after reading docs and the example (https://github.com/arskom/spyne/blob/master/examples/auxproc.py) I was lost (i.e. I had no understanding on whats the point of having two methods present with same name and signature). I think it could be beneficiary to repeat the cited part in the documentation part (i.e. on http://spyne.io/docs/2.10/reference/auxproc.html)