Add specific details about how to map WS Policies to PolicyDocument/PolicyExpression

The foundational spec's "2.4.4 The Policy Model" is extremely vague and provides almost no information. Further, the model itself needs expanded.

For example:

<wsp:ExactlyOne>
<wsp:All>
<!-- assertion a -->
</wsp:All>
<wsp:All>
<!-- assertion b -->
</wsp:All>
</wsp:ExactlyOne>

The ExactlyOne would be a PolicyExpression. But what about the policy "alternatives" within that expression (each "All" node)? Shouldn't concepts like alternatives, nested expressions, assertions, etc. be modeled? Or, if impls are expected to model that on their own through custom attributes, explicitly describe a notional system.