RFR: 8028816: Add value-type notice to Optional* classes

[Mike Duigou]

Hello all;

There's been a discussion on the lambda spec experts list (http://mail.openjdk.java.net/pipermail/lambda-spec-experts/) about adding a notice to the Optional classes about implications of their likely future as values. This discussion recently completed so now there's a doc patch to review:

http://cr.openjdk.java.net/~mduigou/JDK-8028816/0/webrev/

I have already reviewed this but will hold off pushing it for a few hours in case someone notices a mistake that I did not.

Mike