Document that menu item bitmaps must be set before appending them.

At least in MSW we need to know in advance whether we're going to use normal
items or owner-drawn ones currently.

Closes #3641.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72404 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/menuitem.h

@@ -316,6 +316,11 @@ public:
         @a checked is @true (default value) or SetBitmaps(wxNullBitmap, bmp)
         otherwise.
 
+        SetBitmap() must be called before the item is appended to the menu,
+        i.e. appending the item without a bitmap and setting one later is not
+        guaranteed to work. But the bitmap can be changed or reset later if it
+        had been set up initially.
+
         Notice that GTK+ uses a global setting called @c gtk-menu-images to
         determine if the images should be shown in the menus at all. If it is
         off (which is the case in e.g. Gnome 2.28 by default), no images will