RFR: JDK-8305004: add @spec tags to langtools modules

[Jonathan Gibbons]

Please review a simple doc-only PR to add `@spec` tags to 3 "langtools" modules, for eventual use in an "External Specifications" page.

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Commit messages:
 - JDK-8305004: add @spec tags to langtools modules

Changes: https://git.openjdk.org/jdk/pull/13201/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13201&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305004
  Stats: 11 lines in 6 files changed: 5 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13201.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13201/head:pull/13201

PR: https://git.openjdk.org/jdk/pull/13201