fix transparent window example

wxBG_STYLE_TRANSPARENT can only be set before window creation.
closes #16161


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76295 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
update comment for wxBG_STYLE_TRANSPARENT

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76296 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775