RFR: JDK-8320803: Update SourceVersion.RELEASE_22 description for language changes

[Joe Darcy]

Quick doc-only fix.

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Commit messages:
 - JDK-8320803: Update SourceVersion.RELEASE_22 description for language changes

Changes: https://git.openjdk.org/jdk/pull/16832/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16832&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320803
  Stats: 9 lines in 1 file changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16832.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16832/head:pull/16832

PR: https://git.openjdk.org/jdk/pull/16832