Mention "extern types" on the opaque structs section

src/ffi.md

@@ -705,8 +705,14 @@ for more information.
 # Representing opaque structs
 
 Sometimes, a C library wants to provide a pointer to something, but not let you
-know the internal details of the thing it wants. The simplest way is to use a
-`void *` argument:
+know the internal details of the thing it wants. The simplest way is to use "extern types".
+But it's currently (as of June 2021) unstable and has some unresolved questions,
+see the [RFC page][extern-type-rfc] and the [tracking issue][extern-type-issue] for more details.
+
+[extern-type-issue]: https://github.com/rust-lang/rust/issues/43467
+[extern-type-rfc]: https://rust-lang.github.io/rfcs/1861-extern-types.html
+
+Alternatively, we can use a `void *` argument:
 
 ```c
 void foo(void *arg);