[13] RFR 8226963: More clarification on possible sequencing error in GSSContext::unwrap

[Weijun Wang]

Please take a review at

   http://cr.openjdk.java.net/~weijun/8226963/webrev.00/

Looks like some users are not aware of the MessageProp parameter in unwrap and 
forget to check about it. This fix adds some code in the example of the 
GSSContext class spec.

Thanks,
Max