Add comment linking engine creating code together

crates/plugin_runtime/build.rs

@@ -48,7 +48,11 @@ fn main() {
     }
 }
 
-fn create_engine() -> Engine {
+/// Creates a default engine for compiling Wasm.
+/// N.B.: this must create the same `Engine` as
+/// the `create_default_engine` function
+/// in `plugin_runtime/src/plugin.rs`.
+fn create_default_engine() -> Engine {
     let mut config = Config::default();
     config.async_support(true);
     // config.epoch_interruption(true);