RFR: JDK-8298142: Update internal comment on language features in SourceVersion

[Joe Darcy]

Update the comments on the various source-versioning enums on the language changes in recent releases.

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Commit messages:
 - JDK-8298142: Update internal comment on language features in SourceVersion

Changes: https://git.openjdk.org/jdk/pull/11525/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11525&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298142
  Stats: 9 lines in 2 files changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/11525.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11525/head:pull/11525

PR: https://git.openjdk.org/jdk/pull/11525