Proposed additional text: i070 - Clarify relationship between extensibilitymodel and policy

[Prateek Mishra <prateek.mishra@oracle.com>]

Add to Section 11.2:

(include after line 2372)

As explained in Section 3.1.3, an assertion with a nested policy element 
cannot match the same assertion without
a nested policy element. This should be kept in mind when using 
assertions that have been extended with additional
assertions that are meant to be placed within a nested policy element. 
For example, use of an extension assertion
 that indicates the presence or absence of an optional field within a 
security token may result in policy intersection
failure.