[erlang-questions] Documentation improvements for gen_statem (Mostly)

[Raimo Niskanen]

I just wanted to inform you that I have, inspired by feedback from the
Code Beam Conference in San Francisco, created a pull request #1754:
    https://github.com/erlang/otp/pull/1754
with improvements to the User's Guide and how to find it, and a change of
an error exception value when you return an action not allowed in
a state enter call. 

Feedback is welcome!
-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB