[docs] QList: fix history of clear() semantics change

The description was copied from QVector, and doesn't exactly fit
QList. For QList, the behavior changed in Qt 6.0 when QList became
QVector, not in Qt 5.7, as indicated.

Be more precise.

Pick-to: 6.4 6.2
Change-Id: I4029d83128ec205f628125d78394e8ee79cc221f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qlist.qdoc

@@ -549,17 +549,14 @@
 
     Removes all the elements from the list.
 
-    \note Until Qt 5.6, this also released the memory used by
+    If this list is not shared, the capacity() is preserved. Use squeeze() to
-    the list. From Qt 5.7, the capacity is preserved. To shed
+    shed excess capacity.
-    all capacity, swap with a default-constructed list:
-    \code
-    QList<T> l ...;
-    QList<T>().swap(l);
-    Q_ASSERT(l.capacity() == 0);
-    \endcode
-    or call squeeze().
 
-    \sa squeeze()
+    \note In Qt versions prior to 5.7 (for QVector) and 6.0 (for QList), this
+    function released the memory used by the list instead of preserving the
+    capacity.
+
+    \sa resize(), squeeze()
 */
 
 /*! \fn template <typename T> const T &QList<T>::at(qsizetype i) const