net: add comments explaining error check

PR-URL: #24222
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/net.js

@@ -263,9 +263,16 @@ function Socket(options) {
     const { fd } = options;
     let err;
 
+    // createHandle will throw ERR_INVALID_FD_TYPE if `fd` is not
+    // a valid `PIPE` or `TCP` descriptor
     this._handle = createHandle(fd, false);
 
     err = this._handle.open(fd);
+
+    // While difficult to fabricate, in some architectures
+    // `open` may return an error code for valid file descriptors
+    // which cannot be opened. This is difficult to test as most
+    // un-openable fds will throw on `createHandle`
     if (err)
       throw errnoException(err, 'open');