[Nestmates] Updates to the java.lang.Class docs in relation to nests

[David Holmes]

Following some extensive discussion on the JEP-181 CSR, Alex proposed a 
number of changes to the way we described nests and the new nest related 
methods in java.lang.Class. There is no normative change to the new 
reflection API that was added, just a change in how things are presented 
and described. A new overview was added to the class-level javadoc.

For reference the latest version can be seen here:
http://cr.openjdk.java.net/~dholmes/8010319-JEP181/specs/

The updated RFR for these changes was posted here:

http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-June/053913.html

Thanks,
David