Add a comment about how wasmtime-c-api is linked by cargo

lib/binding_rust/wasm_language.rs

@@ -4,6 +4,8 @@ pub use wasmtime;
 
 #[cfg(feature = "wasm")]
 pub fn test() {
+    // Force Cargo to include wasmtime-c-api as a dependency of this crate,
+    // even though it is only used by the C code.
     wasmtime_c_api::wasm_engine_new();
 }