A disclaimer or two for Optional

[Brian Goetz]

> It's fine to warn people in general about the potential pitfalls that
> exist today or that might arise in the future due to new language
> features. It just seems strange to focus on Optional, a class whose name
> practically shouts to the user, "Don't synchronize on me!"

Its not *just* Optional.  For example, the JSR-310 folks are listening 
to this discussion with interest, since all of their new date-time 
classes are morally values, and they would understandably prefer to not 
have them all boxed.

But we should only focus on stuff that's new in 8; the previous classes 
that are morally values are likely too far polluted with existing 
usages.  Its only the new value-ish classes in 8 that we have a fighting 
chance of turning into values in a future version.