use focused bitmap as fallback for the hover one

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@36078 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/bbutton.tex

@@ -40,7 +40,11 @@ to be set.}
 \twocolitem{\windowstyle{focus}}{Bitmap shown when the button has keyboard
 focus but is not pressed.}
 \twocolitem{\windowstyle{hover}}{Bitmap shown when the mouse is over the
-button (but it is not pressed).}
+button (but it is not pressed). Notice that if hover bitmap is not specified
+but the current platform UI uses hover images for the buttons (such as Windows
+XP or GTK+), then the focus bitmap is used for hover state as well. This makes
+it possible to set focus bitmap only to get reasonably good behaviour on all
+platforms.}
 \end{twocollist}