I believe this started when @ejesche said this would made ginga work faster, @taldcroft.  

There is a subtle point here that we are not enforce true immutability at the array level, because one can always do something like ``c1.lat[2] = 3*u.degree`` - that's directly changing the array.  What I'm saying here is that we essentially say "the public API treats these as immutable, so using it this way is not supported and may not work how you expect."  I'll add a note to this effect in the changes I'm making now.