RFR: 8303755: Clean up around JavacElements.getAllMembers

[Pavel Rappo]

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Please review this cleanup which consists of commentary changes and code simplifications.

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Commit messages:
 - Improve CompoundIterator
 - Improve Iterators.createFilterIterator
 - Fix typos and improve consistency
 - Avoid unnecessary recomputation
 - Fix typos and {@inheritDoc} that stands out

Changes: https://git.openjdk.org/jdk/pull/12904/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12904&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303755
  Stats: 51 lines in 6 files changed: 9 ins; 20 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/12904.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12904/head:pull/12904

PR: https://git.openjdk.org/jdk/pull/12904