docs/config: added documentation for auth:user-registration flag

Starting from documentation this time :-)

Related to #380.
api: don't register CreateUser handler when user-registration is off

Related to #380.
cmd: return response on request failures

We need to allow clients to handle their own failures. I'm changing this
because in order to fix issue #380, the userCreate command will need to
handle 404 and 405 failures.
cmd: display friendlier error message when user creation is disabled

Closes #380.