util: improve function signature of util._extend

The function signature of `util._extend` is not intuitive and the
documentation doesn't specify the necessary second parameter. This
patch changes the parameter names in the code and the function params
in doc.

PR-URL: #8187
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

doc/api/util.md

@@ -812,7 +812,7 @@ deprecated: v0.11.3
 
 Deprecated predecessor of `console.log`.
 
-### util.\_extend(obj)
+### util.\_extend(target, source)
 <!-- YAML
 added: v0.7.5
 deprecated: v6.0.0

lib/util.js

@@ -983,16 +983,16 @@ exports.inherits = function(ctor, superCtor) {
   Object.setPrototypeOf(ctor.prototype, superCtor.prototype);
 };
 
-exports._extend = function(origin, add) {
-  // Don't do anything if add isn't an object
-  if (add === null || typeof add !== 'object') return origin;
+exports._extend = function(target, source) {
+  // Don't do anything if source isn't an object
+  if (source === null || typeof source !== 'object') return target;
 
-  var keys = Object.keys(add);
+  var keys = Object.keys(source);
   var i = keys.length;
   while (i--) {
-    origin[keys[i]] = add[keys[i]];
+    target[keys[i]] = source[keys[i]];
   }
-  return origin;
+  return target;
 };
 
 function hasOwnProperty(obj, prop) {