RFR: 8314896: additional clarifications to reversed() default methods' implementation requirements

[Stuart Marks]

Wording changes to make clear that the scenarios described are merely examples and are not normative requirements.

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Commit messages:
 - Update wording for {Deque,List,SortedMap,SortedSet}.reversed implSpecs.

Changes: https://git.openjdk.org/jdk/pull/15799/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15799&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314896
  Stats: 12 lines in 4 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/15799.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15799/head:pull/15799

PR: https://git.openjdk.org/jdk/pull/15799