RFR: 8038146: Clarify NavigableMap/TreeMap support for Entry.setValue

[Stuart Marks]

Also additional verbiage regarding Map.Entry objects in general and their possible connection to an underlying Map.

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Commit messages:
 - 8038146: Clarify NavigableMap/TreeMap support for Entry.setValue

Changes: https://git.openjdk.org/jdk/pull/11956/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11956&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8038146
  Stats: 77 lines in 4 files changed: 41 ins; 3 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/11956.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11956/head:pull/11956

PR: https://git.openjdk.org/jdk/pull/11956