RFR: JDK-8284966: Update SourceVersion.RELEASE_19 description for language changes

Thu May 19 18:02:24 UTC 2022

Updating the description of RELEASE_19 for the anticipated language changes in Java SE 19. I'll defer pushing until the proposed-to-target record patterns compiler changes are pushed. Also adding links to the JSR 269 API in the package info files.

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Commit messages:
 - JDK-8284966: Update SourceVersion.RELEASE_19 description for language changes

Changes: https://git.openjdk.java.net/jdk/pull/8793/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8793&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284966
  Stats: 19 lines in 6 files changed: 16 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8793.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8793/head:pull/8793

PR: https://git.openjdk.java.net/jdk/pull/8793