Apply suggestions from code review

Co-authored-by: Matteo Collina <matteo.collina@gmail.com>
Co-authored-by: Ben Noordhuis <info@bnoordhuis.nl>

Author: 3 people

doc/api/cli.md

@@ -723,7 +723,7 @@ Define which module system, `module` or `commonjs`, to use for the following:
 
 In other words, `--experimental-type=module` flips all the places where Node.js
 currently defaults to CommonJS to instead default to ECMAScript modules, with
-the exception of packages inside `node_modules`.
+the exception of packages inside `node_modules`, for backward compatibility reasons.
 
 Under `--experimental-type=module` and `--experimental-wasm-modules`, files with
 no extension will be treated as WebAssembly if they begin with the WebAssembly

lib/internal/modules/esm/formats.js

@@ -56,7 +56,7 @@ function getFormatOfExtensionlessFile(url) {
       return 'wasm';
     }
   } finally {
-    if (fd) { closeSync(fd); }
+    if (fd !== undefined) { closeSync(fd); }
   }
 
   return 'module';