RFR: JDK-8305593: Add @spec tags in java.desktop

[Jonathan Gibbons]

Please review a docs-only change to add `@spec` tags into `java.desktop` public API files

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Commit messages:
 - JDK-8305593: Add @spec tags in java.desktop

Changes: https://git.openjdk.org/jdk/pull/13360/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13360&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305593
  Stats: 32 lines in 13 files changed: 29 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13360.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13360/head:pull/13360

PR: https://git.openjdk.org/jdk/pull/13360