tutorial: Mention by-value argument passing style.

Author: erickt

doc/tutorial/args.md

@@ -104,6 +104,9 @@ Another style is by-move, which will cause the argument to become
 de-initialized on the caller side, and give ownership of it to the
 called function. This is written `-`.
 
+Sometimes you need to pass a structural type by value, such as when
+interfacing with external native functions. This is written `++`.
+
 Finally, the default passing styles (by-value for non-structural
 types, by-reference for structural ones) are written `+` for by-value
 and `&&` for by(-immutable)-reference. It is sometimes necessary to