deny(missing_docs) at crate root

To help flag missing documentation on publicly accessible APIs.
diff --git a/src/exception.rs b/src/exception.rs
index e0c2f1e..2ae470e 100644
--- a/src/exception.rs
+++ b/src/exception.rs
@@ -16,6 +16,7 @@
 impl std::error::Error for Exception {}
 
 impl Exception {
+    #[allow(missing_docs)]
     pub fn what(&self) -> &str {
         &self.what
     }