net: add comments explaining error check

PR-URL: #24222 Reviewed-By: Ruben Bridgewater <[email protected]> Reviewed-By: Luigi Pinca <[email protected]>

Author: bewchy

lib/net.js

@@ -264,9 +264,16 @@ function Socket(options){
 const{ fd }=options;
 leterr;
 
+// 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)
 throwerrnoException(err,'open');