[lworld] RFR: 8346307: [lworld] Document WeakHashMap and other apis that may throw IdentityException [v3]

[Roger Riggs]

> Add APINote  and javadoc for IdentityException where it will be useful to know that identity or value objects are treated differently. 
> Simplified WeakHashMap javadoc updates for IdentityException. 
> Added note to System.identityHashCode to include value objects.
> Added to class javadoc for IdentityHashMap for value objects.

Roger Riggs has updated the pull request incrementally with two additional commits since the last revision:

 - Merge remote-tracking branch 'refs/remotes/origin/8346307-throws-identityexception' into 8346307-throws-identityexception
 - 8346307: [lworld] Document WeakHashMap and other apis that may throw IdentityException
   Add APINote and javadoc for IdentityException where it will be useful to know.
   Updated Objects.hasIdentity to return true for non-null reference to identity class; else false
   Added Objects.isValueObject to return true for non-null reference to value class; else false
   Updated tests for value objects.

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1327/files
  - new: https://git.openjdk.org/valhalla/pull/1327/files/2aae8e67..850923bd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1327&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1327&range=01-02

  Stats: 141 lines in 8 files changed: 77 ins; 21 del; 43 mod
  Patch: https://git.openjdk.org/valhalla/pull/1327.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1327/head:pull/1327

PR: https://git.openjdk.org/valhalla/pull/1327