RFR: JDK-8284928: Add links from SourceVersion to specific JLS versions

[Joe Darcy]

As a convenience to readers, add links from the SourceVersion constants to the particular JLS release they correspond to.

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Commit messages:
 - JDK-8284928: Add links from SourceVersion to specific JLS versions

Changes: https://git.openjdk.java.net/jdk/pull/8275/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8275&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284928
  Stats: 52 lines in 1 file changed: 52 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8275.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8275/head:pull/8275

PR: https://git.openjdk.java.net/jdk/pull/8275