xdg-shell: document resize with "none" edges

Current compositor behavior here is not terribly consistent:

Both Weston and Mutter ignore resize requests with "none" edges.

wlroots currently kills the client with a protocol error but allowed compositors using the wlroots library to decide what to do in the past before this behavior was accidentally changed. Sending a protocol here is a wlroots bug in my opinion since the none enum variant exists and the invalid_resize_edge protocol error is only specified to be sent for values that do not match an enum variant.

KWin seems to treat a resize with "none" edges the same as a move request, which is quite strange. I would consider this a bug as well.

This patch recommends that compositors ignore resize requests with "none" edges in the interest of increasing client portability.