QList: also default count()'s template parameter

Also fix the docs.

Change-Id: If08116cb8657d00d610de9451be0ba73ce19dcd1
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/tools/qlist.h

@@ -352,7 +352,7 @@ public:
     bool contains(const AT &t) const noexcept;
 #endif
 
-    template <typename AT>
+    template <typename AT = T>
     qsizetype count(const AT &t) const noexcept
     {
         return qsizetype(std::count(&*cbegin(), &*cend(), t));

src/corelib/tools/qlist.qdoc

@@ -1013,7 +1013,7 @@
 */
 
 
-/*! \fn template <typename T> qsizetype QList<T>::count(parameter_type value) const
+/*! \fn template <typename T> template <typename AT = T> qsizetype QList<T>::count(const AT &value) const
 
     Returns the number of occurrences of \a value in the list.