(PUP-511) Remove Non-Value expressions

This removes the final references to non-value producing expressions.
(PUP-511) Specify resource expressions

This brings the specification of resource expressions in line with the
work done for PUP-2898 as well as clarifying some of the semantic
constraints on the expressions (no duplicate titles, no duplicate
attributes, etc.)
(maint) Add information for Default type

The default expression was given the type Default, but we forgot to add
it to the language specification.