Deny missing documentation in wasmtime crate (#836)

* Deny missing documentation in `wasmtime` crate

Everything is largely documented now, so let's be sure to keep it that
way!

* Add windows docs

crates/api/src/lib.rs

@@ -6,6 +6,8 @@
 //! and there to implement Rust idioms. This crate also defines the actual C API
 //! itself for consumption from other languages.
 
+#![deny(missing_docs)]
+
 mod callable;
 mod externals;
 mod instance;