deny(missing_docs) at crate root

To help flag missing documentation on publicly accessible APIs.
diff --git a/src/function.rs b/src/function.rs
index 647be6a..f0a2010 100644
--- a/src/function.rs
+++ b/src/function.rs
@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 use core::ffi::c_void;
 
 #[repr(C)]