jQuery.extend: Separate the one-argument signature from the multiple-arguments one

This is done to reduce the confusion as the single-argument signature behaves
completely differently than the other one.

Ref jquery/jquery#4748
Fixes jquerygh-1164
Closes jquerygh-1170

Author: mgol

entries/jQuery.extend.xml

@@ -4,9 +4,9 @@
   <signature>
     <added>1.0</added>
     <argument name="target" type="Object">
-      <desc> An object that will receive the new properties if additional objects are passed in or that will extend the jQuery namespace if it is the sole argument.</desc>
+      <desc> An object that will receive the new properties.</desc>
     </argument>
-    <argument name="object1" type="Object" optional="true">
+    <argument name="object1" type="Object">
       <desc>An object containing additional properties to merge in.</desc>
     </argument>
     <argument name="objectN" optional="true" type="Object">
@@ -28,6 +28,12 @@
       <desc>Additional objects containing properties to merge in.</desc>
     </argument>
   </signature>
+  <signature>
+    <added>1.0</added>
+    <argument name="object" type="Object">
+      <desc> An object to merge onto the jQuery namespace.</desc>
+    </argument>
+  </signature>
   <desc>Merge the contents of two or more objects together into the first object.</desc>
   <longdesc>
     <p>When two or more object arguments are supplied to <code>$.extend()</code>, properties from all of the objects are added to the target object. Arguments that are <code>null</code> or <code>undefined</code> are ignored.</p>