attr: Document attr(key, null) to remove attribute

Fixes gh-523
Closes gh-853

Author: jugglinmike

entries/attr.xml

@@ -147,7 +147,8 @@ The title of the emphasis is:<div></div>
       <argument name="value">
         <type name="String"/>
         <type name="Number"/>
-        <desc>A value to set for the attribute.</desc>
+        <type name="Null"/>
+        <desc>A value to set for the attribute. If <code>null</code>, the specified attribute will be removed (as in <a href="/removeAttr/"><code>.removeAttr()</code></a>).</desc>
       </argument>
     </signature>
     <signature>

pages/Types.html

@@ -432,6 +432,9 @@ <h2 id="PlainObject"> PlainObject </h2>
   jQuery.isPlainObject( o ); // true
 </code></pre>
 
+<h2 id="Null">Null</h2>
+<p>The <code>null</code> keyword is a JavaScript literal that is commonly used to express the absence of an intentional value.</p>
+
 <h2 id="Date"> Date </h2>
 <p>The Date type is a JavaScript object that represents a single moment in time. Date objects are instantiated using their constructor function, which by default creates an object that represents the current date and time.
 </p>