RFR: JDK-8309884: missing @since tags in `com.sun.source.*`

[Jonathan Gibbons]

Please review a trivial docs fix to address missing `@since` tags in `com.sun.source` API.

-------------

Commit messages:
 - JDK-8309884: missing @since tags in `com.sun.source.*`

Changes: https://git.openjdk.org/jdk/pull/14434/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14434&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309884
  Stats: 7 lines in 4 files changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14434.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14434/head:pull/14434

PR: https://git.openjdk.org/jdk/pull/14434