Reflex.Host: more superclass constraints + docs
Reflex.Host: more superclass constraints + docs

Just as Reflex has superclass constraints for MonadSample t (PushM t),
ReflexHost should also have some superclass constraints for its
associated types. This commit adds the following constraints for
`HostFrame t`: `MonadReflexCreateTrigger t, MonadSample t, MonadHold t`.

This commit also adds docs for the functions in the Reflex.Host.Class
module.

Saulzar/reflex,reflex-frp/reflex,Saulzar/reflex,reflex-frp/reflex