RFR: 8339696: Clarify modeling scope of javax.lang.model.element

[Joe Darcy]

Clarify that while javax.lang.model does not include an AST API, it is intended to be usable by an AST API.

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Commit messages:
 - Appease jcheck.
 - JDK-8339696: Clarify modeling scope of javax.lang.model.element

Changes: https://git.openjdk.org/jdk/pull/20900/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20900&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339696
  Stats: 10 lines in 1 file changed: 6 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20900.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20900/head:pull/20900

PR: https://git.openjdk.org/jdk/pull/20900