documents nqp::push return value not defined

In the quest for documenting the return value for moarvm nqp::push some
discrepancies were discovered...
TimToady> I guess moar is just returning the 2
> which is kinda not what I'd expect
> I don't see anywhere in nqp where the value is used, actually
> er, in nom/src I should say
> might be best to say the return value is currently undefined