Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 property_constraint_pv(Arg1, Arg2, Arg3, Arg4)

 var_drug_condition(Arg1, Arg2, Arg3, Arg4)

 enlabel(Arg1, Arg2)

 enlabelp(Arg1, Arg2)

 enlabel_any(Arg1, Arg2)

 exact_match(Arg1, Arg2)