Correct information about all headers being movable.

This only applies to QTreeView.

8eb3d724a9 introduced this error.

Change-Id: Ia8a2c387afbd19e4d98ea6f81e5ce6409d79bcd9
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qheaderview.cpp

@@ -1087,9 +1087,6 @@ 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()
 */