docs: clarify that implementations may add additional arguments (#870)

Author: mdhaber

spec/draft/purpose_and_scope.md

@@ -410,8 +410,8 @@ all the functions, arguments, data types, syntax, and semantics described in
 this specification.
 
 A conforming implementation of the array API standard may provide additional
-values, objects, properties, data types, and functions beyond those described
-in this specification.
+features (e.g., values, objects, properties, data types, functions, and function
+arguments) beyond those described in this specification.
 
 Libraries which aim to provide a conforming implementation but haven't yet
 completed such an implementation may, and are encouraged to, provide details on