RFR: 8199318: add idempotent copy operation for Map.Entry [v2]

[Stuart Marks]

> I'm fixing this along with a couple intertwined issues.
> 
> 1. Add Map.Entry::copyOf as an idempotent copy operation.
> 
> 2. Clarify that AbstractMap.SimpleImmutableEntry is itself unmodifiable (not really immutable) but that subclasses can be modifiable.
> 
> 3. Clarify some confusing, historical wording about Map.Entry instances being obtainable only via iteration of a Map's entry-set view. This was never really true, since anyone could implement the Map.Entry interface, and it certainly was not true since JDK 1.6 when SimpleEntry and SimpleImmutableEntry were added.

Stuart Marks has updated the pull request incrementally with one additional commit since the last revision:

  Tiny editorial tweaks.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4295/files
  - new: https://git.openjdk.java.net/jdk/pull/4295/files/841a154c..c67b6445

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4295&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4295&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4295.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4295/head:pull/4295

PR: https://git.openjdk.java.net/jdk/pull/4295