jQuery.ajax: Document special handling of a double question mark

Fixes gh-999
Ref gh-1005

Co-authored-by: Dave Methvin <[email protected]>

Author: mgol

entries/jQuery.ajax.xml

@@ -84,7 +84,11 @@ $.ajax({
         <type name="PlainObject"/>
         <type name="String"/>
         <type name="Array"/>
-        <desc>Data to be sent to the server. It is converted to a query string, if not already a string. It's appended to the url for GET-requests. See <code>processData</code> option to prevent this automatic processing. Object must be Key/Value pairs. If value is an Array, jQuery serializes multiple values with same key based on the value of the <code>traditional</code> setting (described below).</desc>
+        <desc>
+            <p>Data to be sent to the server. If the HTTP method is one that cannot have an entity body, such as GET, the <code>data</code> is appended to the URL.</p>
+            <p>When <code>data</code> is an object, jQuery generates the data string from the object's key/value pairs unless the <code>processData</code> option is set to <code>false</code>. For example, <code>{ a: "bc", d: "e,f" }</code> is converted to the string <code>"a=bc&amp;d=e%2Cf"</code>. If the value is an array, jQuery serializes multiple values with same key based on the value of the <code>traditional</code> setting (described below). For example, <code>{ a: [1,2] }</code> becomes the string <code>"a%5B%5D=1&amp;a%5B%5D=2"</code> with the default <code>traditional: false</code> setting.</p>
+            <p>When <code>data</code> is passed as a string it should <strong>already be encoded</strong> using the correct encoding for <code>contentType</code>, which by default is <code>application/x-www-form-urlencoded</code>. If the string contains a double question mark (<code>??</code>) it is replaced with a value generated by jQuery that is unique for each copy of the library on the page (e.g. <code>jQuery21406515378922229067_1479880736745</code>).</p>
+        </desc>
       </property>
       <property name="dataFilter" type="Function">
         <argument name="data" type="String"/>