doc: clarify nullability information source for empty literals

proto/substrait/algebra.proto

@@ -819,9 +819,12 @@ message Expression {
  UserDefined user_defined = 33;
  }
 
- // whether the literal type should be treated as a nullable type. Applies to
- // all members of union other than the Typed null (which should directly
- // declare nullability).
+ // Whether the literal_type above should be treated as a nullable type.
+ // Applies to all members of the literal_type oneof EXCEPT:
+ // * Type null
+ // * Type.List empty_list
+ // * Type.Map empty_map
+ // which should declare their nullability directly instead.
  bool nullable = 50;
 
  // optionally points to a type_variation_anchor defined in this plan.