[DomCrawler] Warn users of older PHP versions Crawler might not decode html entities properly.

Author: jakzal

components/dom_crawler.rst

@@ -231,6 +231,7 @@ and :phpclass:`DOMNode` objects:
         $html = $crawler->html();
 
     The ``html`` method is new in Symfony 2.3.
+    In PHP < 5.3.6 it might return html entities which are not properly decoded.
 
 Links
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