Add Result to builtin types table

Closes #53.
diff --git a/src/lib.rs b/src/lib.rs
index 5dd2649..b03c234 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -306,6 +306,7 @@
 //! <tr><td><a href="https://docs.rs/cxx/0.2/cxx/struct.CxxString.html">CxxString</a></td><td>std::string</td><td><sup><i>cannot be passed by value</i></sup></td></tr>
 //! <tr><td>Box&lt;T&gt;</td><td>rust::Box&lt;T&gt;</td><td><sup><i>cannot hold opaque C++ type</i></sup></td></tr>
 //! <tr><td><a href="https://docs.rs/cxx/0.2/cxx/struct.UniquePtr.html">UniquePtr&lt;T&gt;</a></td><td>std::unique_ptr&lt;T&gt;</td><td><sup><i>cannot hold opaque Rust type</i></sup></td></tr>
+//! <tr><td>Result&lt;T&gt;</td><td>error ↔️ exception</td><td><sup><i>allowed as return type only</i></sup></td></tr>
 //! </table>
 //!
 //! The C++ API of the `rust` namespace is defined by the *include/cxx.h* file