RFR: 8345065: Cleanup DomainCombiner, SubjectDomainCombiner, Subject, and PrivilegedAction specifications

[Sean Mullan]

A few additional changes should be made to the API specs for these classes .  These changes are documenting behavior of other APIs that has already been approved as part of JEP 486, so I don't think a CSR is necessary, but opinions on that are welcome.

The `DomainCombiner` and `SubjectDomainCombiner` classes contain wording about "inherited" and"privileged" `ProtectionDomain`s which is no longer  implemented, and so has been removed. The `PrivilegedAction` classes also contained wording about "privileged" computations which is no longer accurate. I also added an API note to these classes, similar to the ones in the `Permission` subclasses. Finally, I removed a couple of instances of "privileged work" from the `Subject` class which no longer applies.

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Commit messages:
 - Remove trailing whitespace.
 - Initial cleanup.

Changes: https://git.openjdk.org/jdk/pull/22500/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22500&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345065
  Stats: 35 lines in 6 files changed: 8 ins; 8 del; 19 mod
  Patch: https://git.openjdk.org/jdk/pull/22500.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22500/head:pull/22500

PR: https://git.openjdk.org/jdk/pull/22500