Add note about RVO in cpp.md
`bindgen`'s inability to currently support RVO should be noted in C++ bindings generation document.
diff --git a/book/src/cpp.md b/book/src/cpp.md
index 1be7a8a..75ef855 100644
--- a/book/src/cpp.md
+++ b/book/src/cpp.md
@@ -71,3 +71,8 @@
generate undefined behaviour. See
[the tracking issue for exceptions](https://github.com/rust-lang/rust-bindgen/issues/1208)
for more details.
+
+* Return value optimization. C++ compilers will in certain circumstances optimize functions
+ returning a struct type value to instead take an extra hidden argument that refers
+ to the return value struct. `bindgen` cannot necessarily know about this optimization and
+ thus at present `bindgen`-interfaces for these kinds of functions are invalid.
