RFR: 8308694: Clarify reversed() default methods' implementation requirements

[Stuart Marks]

Can I get a preliminary review of the wording for Deque.reversed()? If the text is good, I'll make corresponding changes to the implSpecs of the other reversed() default methods, namely those in List, SortedMap, and SortedSet and then file the CSR.

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Commit messages:
 - 8308694: Clarify reversed() default methods' implementation requirements

Changes: https://git.openjdk.org/jdk/pull/14504/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14504&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308694
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14504.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14504/head:pull/14504

PR: https://git.openjdk.org/jdk/pull/14504