RFR: 8310991: missing @since tags in java.xml

[Joe Wang]

Please review a patch that adds the @since tag to DOM package.

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Commit messages:
 - 8310991: missing @since tags in java.xml

Changes: https://git.openjdk.org/jdk/pull/15109/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15109&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310991
  Stats: 36 lines in 18 files changed: 36 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15109.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15109/head:pull/15109

PR: https://git.openjdk.org/jdk/pull/15109