RFR: JDK-8284923: Update description of SourceVersion.RELEASE_18

[Joe Darcy]

The description of SourceVersion.RELEASE_18 does not explicitly state what language changes are present, or not present in the release.

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Commit messages:
 - JDK-8284923: Update description of SourceVersion.RELEASE_18

Changes: https://git.openjdk.java.net/jdk/pull/8268/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8268&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284923
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8268.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8268/head:pull/8268

PR: https://git.openjdk.java.net/jdk/pull/8268