Update key-concepts with XDomainRequest limitation

Added this update in order clarify the understanding implied in the sentence 'This limitation does not apply to scripts that are loaded via jQuery's Ajax methods'. This will help the beginner to understand $.ajax does not work seamlessly with every browser, but only with help of some plugins.

Author: moo3

page/ajax/key-concepts.md

@@ -71,7 +71,9 @@ $.get( "foo.php", function( response ) {
 
 ### Same-Origin Policy and JSONP
 
-In general, Ajax requests are limited to the same protocol (http or https), the same port, and the same domain as the page making the request. This limitation does not apply to scripts that are loaded via jQuery's Ajax methods.
+In general, Ajax requests are limited to the same protocol (http or https), the same port, and the same domain as the page making the request. This limitation does not apply to scripts that are loaded via jQuery's Ajax methods, except for Internet Explorer 8 and 9. 
+
+Since Internet Explorer versions 8 and 9 uses [XDomainRequest](http://msdn.microsoft.com/en-us/library/ie/cc288060(v=vs.85).aspx), which has only limited support to XHR2 implementation. Further reading of [this ticket](http://bugs.jquery.com/ticket/8283) would help understand the relationship between XDomainRequest and jQuery.
 
 The other exception is requests targeted at a JSONP service on another domain. In the case of JSONP, the provider of the service has agreed to respond to your request with a script that can be loaded into the page using a `<script>` tag, thus avoiding the same-origin limitation; that script will include the data you requested, wrapped in a callback function you provide.