After discussing with Mitch and understanding better the X bitmap/pixmap
history, I make the warning more specific to X bitmap cursors only (not
pixmap).
Also I can see our code always exports RGBA data, so I am not quite sure
if this warning even makes sense since X bitmaps are bicolor maps. On
the other hand, Mitch tells me that "these days gdk turns pixbufs into
bitmaps if the x server doesn't support rgba cursors", so maybe that can
still be of use to warn cursor designers for max compatibility.
Still that's pretty old compatibility stuff, so let's replace "may" by
"might".