Improve documentation regarding moving item view columns.

The only way I was able to find information on this was through bug
reports.

Change-Id: Iac59a82fab9c216495860ad3b1efad1125130002
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/widgets/itemviews/qheaderview.cpp

@@ -1087,6 +1087,9 @@ int QHeaderView::logicalIndex(int visualIndex) const
     If \a movable is true, the header may be moved by the user; otherwise it
     is fixed in place.
 
+    By default, all sections are movable except the first, which always stays
+    fixed.
+
     \sa sectionsMovable(), sectionMoved()
 */
 

src/widgets/itemviews/qtreeview.cpp

@@ -109,6 +109,10 @@ QT_BEGIN_NAMESPACE
     its header. If this value is set to true, this property will override the
     resize mode set on the last section in the header.
 
+    By default, all columns in a tree view are movable except the first. To
+    disable movement of these columns, use QHeaderView's
+    \l {QHeaderView::}{setSectionsMovable()} function. For more information
+    about rearranging sections, see \l {Moving Header Sections}.
 
     \section1 Key Bindings