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

interface/wx/bitmap.h

@@ -91,6 +91,12 @@ public:
         Loads a bitmap from a file or resource, putting the resulting data into
         @a bitmap.
 
+        @note Under MSW, when loading a bitmap from resources (i.e. using @c
+            wxBITMAP_TYPE_BMP_RESOURCE as @a type), the light grey colour is
+            considered to be transparent, for historical reasons. If you want
+            to handle the light grey pixels normally instead, call
+            SetMask(NULL) after loading the bitmap.
+
         @param bitmap
             The bitmap object which is to be affected by this operation.
         @param name