RFR: 8311006: missing @since info in jdk.xml.dom

[Joe Wang]

Please review a patch that add `@since` tags following existing convention for XML APIs in java.xml and jdk.xml.dom modules.

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

Commit messages:
 - 8311006: missing @since info in jdk.xml.dom

Changes: https://git.openjdk.org/jdk/pull/15129/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15129&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311006
  Stats: 12 lines in 6 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15129.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15129/head:pull/15129

PR: https://git.openjdk.org/jdk/pull/15129