documentation pointers to known validator errors due to spec issues

[karlnaden]

Summary

Address known extension context issues that cause spurious validation errors through documentation (issue: #11)

Testing Guidance

1. Review all PAS Client and Server tests and check that in any tests involving validation of a response bundle (submit or inquiry), the documentation ("about" tab) includes a "limitations" section that links back to this issue.
2. Review the suite documentation to verify that there is a "known issues" section there as well pointing to a list of issues tagged with the "source ig issue" label ( source ig issue Relates to a known issue in the DTR IG ) - this list currently includes one issue.