RFR: 8257776: [valhalla:jep390] Add disclaimer about future changes to value-based classes [v3]

[Dan Smith]

> Addressing CSR feedback by mentioning the possibility of future behavioral changes to value-based classes.

Dan Smith has updated the pull request incrementally with one additional commit since the last revision:

  Add synchronization example to ValueBased.html

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Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/294/files
  - new: https://git.openjdk.java.net/valhalla/pull/294/files/bacb42ee..f35d5acf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=294&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=294&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/294.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/294/head:pull/294

PR: https://git.openjdk.java.net/valhalla/pull/294