Review: release note for JDK-8015081

[Jamil Nimeh]

Hello all,

Please review this one release note that documents a change in behavior 
for the Subject class and it's underlying SecureSet collections:

Original bug: https://bugs.openjdk.java.net/browse/JDK-8015081
Release note: https://bugs.openjdk.java.net/browse/JDK-8173069

Text of the release note:
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Inputs to Subject class now prohibit null values in the constructors and 
modification operations on the Principal and credential Set objects 
returned by Subject methods.

For the non-default constructor, the principals, pubCredentials, and 
privCredentials parameters may not be null, nor may any element within 
the Sets be null. A NullPointerException will be thrown if null values 
are provided.

For operations performed on Set objects returned by getPrincipals(), 
getPrivateCredentials() and getPublicCredentials(), a 
NullPointerException is thrown under the following conditions:
* add(), remove(), or contains() provides a null parameter.
* addAll(), removeAll(), containsAll() or retainsAll() provides a 
Collection containing a null element.
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Thanks,
--Jamil