RFR: 8285470: Improve handling of @inheritDoc

[Pavel Rappo]

The only taglet that along with its own tag needs to know the immediately enclosing tag is `InheritDocTaglet`:

    @return {@inheritDoc}
    @throws NullPointerException {@inheritDoc}
    @param p {@inheritDoc}

However, the immediately enclosing tag is unconditionally passed to all taglets. If we stop passing it and make `InheritDocTaglet` compute it instead, the code becomes cleaner.

While reviewing, particularly note these benefits of the proposed change:

 * taglet-handling code knows less about `@inheritDoc`, and
 * `InheritDocTaglet` receives its own tag, not the tag that encloses it

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Commit messages:
 - (cleanup) Simplify retrieveInheritedDocumentation
 - (cleanup) Clarify retrieveInheritedDocumentation
 - (cleanup) Unify specs of commentTagsToContent
 - Stop passing "holderTag"
 - (cleanup) Remove useless null check

Changes: https://git.openjdk.java.net/jdk/pull/8361/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8361&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285470
  Stats: 74 lines in 7 files changed: 6 ins; 26 del; 42 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8361.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8361/head:pull/8361

PR: https://git.openjdk.java.net/jdk/pull/8361