RFR: 8256993: Clarify Package::isSealed javadoc about package sealing vs sealed class or interface

[Mandy Chung]

This adds API note in `Package::isSealed` javadoc to clarify package sealing vs sealed class or interface.   Since it's spec clarification, CSR is not strictly needed.

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Commit messages:
 - make it api Note
 - 8256993: Clarify Package::isSealed javadoc about package sealing vs sealed class or interface

Changes: https://git.openjdk.java.net/jdk/pull/1419/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1419&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256993
  Stats: 16 lines in 1 file changed: 15 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1419.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1419/head:pull/1419

PR: https://git.openjdk.java.net/jdk/pull/1419