RFR: JDK-8246280: Refine API to model sealed classes and interfaces in javax.lang.model

[Joe Darcy]

Simple specification clarification.

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Commit messages:
 - Appease jcheck.
 - JDK-8246280: Refine API to model sealed classes and interfaces in javax.lang.model

Changes: https://git.openjdk.org/jdk/pull/15795/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15795&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8246280
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15795.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15795/head:pull/15795

PR: https://git.openjdk.org/jdk/pull/15795