[docs] Fix reverse STL iteration example

Use reverse_iterator, now that we finally have it.

Change-Id: If74ead1a6075c5437c1d111206913481a495a014
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/doc/snippets/code/doc_src_containers.cpp

@@ -156,10 +156,9 @@ for (i = list.begin(); i != list.end(); ++i)
 QList<QString> list;
 list << "A" << "B" << "C" << "D";
 
-QList<QString>::iterator i = list.end();
-while (i != list.begin()) {
-    --i;
-    *i = (*i).toLower();
+QList<QString>::reverse_iterator i;
+for (i = list.rbegin(); i != list.rend(); ++i)
+    *i = i->toLower();
 }
 //! [11]
 

src/corelib/doc/src/containers.qdoc

@@ -472,9 +472,7 @@
 
     \image stliterators1.png
 
-    Iterating backward with an STL-style iterator requires us to
-    decrement the iterator \e before we access the item. This
-    requires a \c while loop:
+    Iterating backward with an STL-style iterator is done with reverse iterators:
 
     \snippet code/doc_src_containers.cpp 11