For historical/compatibility reasons we handle this colour as transparent in wxMSW when loading the bitmaps from resources, but this is rather not obvious for the new users, so document it and provide a way of compensating for this in the documentation as well. Closes #15371. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74641 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775