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

[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.

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Commit messages:
 - 8346307: [lworld] Document WeakHashMap and other apis that may throw IdentityException

Changes: https://git.openjdk.org/valhalla/pull/1327/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1327&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346307
  Stats: 131 lines in 6 files changed: 123 ins; 0 del; 8 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