doc: clarify nullability information source for empty literals

proto/substrait/algebra.proto

@@ -825,9 +825,11 @@ 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 (must be nullable by definition)
+ // * Type.List empty_list (use Type.List::nullability)
+ // * Type.Map empty_map (use Type.Map::nullability)
  bool nullable = 50;
 
  // optionally points to a type_variation_anchor defined in this plan.