RFR: 8273175: Add @since tags to the DocTree.Kind enum constants

[Pavel Rappo]

Please review this trivial change to update `@since` tags.

Here are the links that I used for comparison; you might also find them useful when reviewing:

  1. https://docs.oracle.com/javase/8/docs/api/index.html (DocTree.Kind was not documented in JDK 8)
  2. https://docs.oracle.com/javase/9/docs/api/com/sun/source/doctree/DocTree.Kind.html
  3. https://docs.oracle.com/javase/10/docs/api/com/sun/source/doctree/DocTree.Kind.html
  4. https://docs.oracle.com/en/java/javase/11/docs/api/jdk.compiler/com/sun/source/doctree/DocTree.Kind.html (for control, to ensure that nothing has changed since JDK 10)
  5. https://docs.oracle.com/en/java/javase/12/docs/api/jdk.compiler/com/sun/source/doctree/DocTree.Kind.html
  6. https://download.java.net/java/early_access/jdk18/docs/api/jdk.compiler/com/sun/source/doctree/DocTree.Kind.html

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6743/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6743&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273175
  Stats: 80 lines in 1 file changed: 80 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6743.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6743/head:pull/6743

PR: https://git.openjdk.java.net/jdk/pull/6743