RFR: 8283864: Clean up DocFinder and friends

[Pavel Rappo]

A clean-up to facilitate more clean-up in the future.

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Commit messages:
 - Remove extra list wrapping
 - Fix DocFinder.Output.toString comment
 - Clarify internal comment (2)
 - Clarify internal comment
 - Cast to ExecutableElement once
 - Remove an outdated note
 - Clean up what was overlooked in JDK-8284299
 - Assert warning context
 - Simplify impl of CommentHelper.getTags
 - Update doc comment for InheritDocTaglet
 - ... and 8 more: https://git.openjdk.java.net/jdk/compare/70251b06...50c33364

Changes: https://git.openjdk.java.net/jdk/pull/8229/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8229&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283864
  Stats: 213 lines in 11 files changed: 54 ins; 50 del; 109 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8229.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8229/head:pull/8229

PR: https://git.openjdk.java.net/jdk/pull/8229