Document light grey colour handling in wxMSW wxBitmap::LoadFile().

Document light grey colour handling in wxMSW wxBitmap::LoadFile().

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