[records] spec clarifications from thread in amber-spec-experts

[John Rose]

The spec-experts group seems to have reasonable consensus
on some clarifications to the javadoc in Record.java.

http://mail.openjdk.java.net/pipermail/amber-spec-experts/2020-January/001985.html

I’ve rebased the webrev to the current tip:

http://cr.openjdk.java.net/~jrose/draft/record-contract

Shall I push this, or will someone else do so?

— John