[xacml] Functions Document again.

"and", "or" and "n-of" now hwave the semantics of their "ordered-*"
counter parts, and the "order-*" names have been removed.

Two new functions for catching the ERROR condition.

type-on-error

type-sequence-on-error

Each of which take two arguments, of which the second argument is a
"default" value for the expression should evaluating the first argument
result in ERROR.

We still need specifications for all the matching and equals functions!

Cheers,
-Polar