RFR: 8307840: SequencedMap view method specification and implementation adjustments

[Stuart Marks]

Adjust the specification of the `SequencedMap` sequenced-view methods, and adjust implementations to match.

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Commit messages:
 - Minor wording adjustment.
 - 8307840: SequencedMap view method specification and implementation adjustments

Changes: https://git.openjdk.org/jdk/pull/14267/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14267&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307840
  Stats: 113 lines in 3 files changed: 64 ins; 3 del; 46 mod
  Patch: https://git.openjdk.org/jdk/pull/14267.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14267/head:pull/14267

PR: https://git.openjdk.org/jdk/pull/14267