Resolve incomplete documentation for Responsys

Brandon McConnell - PRB

Refine the Responsys documentation to include all built-ins (e.g. ?string, ?has_content, ?is_directive), as some are currently missing.

Some like ?string are included in the documentation but only regarding converting a type into the default string type. ?string in its current state in Responsys also supports the following logic, similar to a ternary:

someCondition?string(whenTrue, whenFalse)

These additional syntaxes would be very important and helpful to include in the documentation.