Foreign API documentation and small cleanup (#31)

* verify slate messages documentation

* rustfmt

* foreign API documentation

* rustfmt

impls/src/test_framework/testclient.rs

@@ -217,7 +217,7 @@ where
 			let mut w = wallet.1.lock();
 			w.open_with_credentials()?;
 			// receive tx
-			foreign::receive_tx(&mut *w, &mut slate, None, None, false)?;
+			slate = foreign::receive_tx(&mut *w, &slate, None, None, false)?;
 			w.close()?;
 		}