RFR: JDK-8308388: Update description of SourceVersion.RELEASE_21 [v2]

[Joe Darcy]

> Update description of SourceVersion.RELEASE_21 for the new language features (once they are pushed).

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14051/files
  - new: https://git.openjdk.org/jdk/pull/14051/files/6a376493..12754063

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14051&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14051&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14051.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14051/head:pull/14051

PR: https://git.openjdk.org/jdk/pull/14051