Add warning about bed velocity

mods/ITEMS/mcl_beds/functions.lua

@@ -83,6 +83,9 @@ local function lay_down(player, pos, bed_pos, state, skip)
 		end
 
 		-- No sleeping while moving. Slightly different behaviour than in MC.
+		-- FIXME: Velocity threshold should be 0.01 but Minetest 5.3.0
+		-- sometimes reports incorrect Y speed. A velocity threshold
+		-- of 0.125 still seems good enough.
 		if vector.length(player:get_player_velocity()) > 0.125 then
 			minetest.chat_send_player(name, S("You have to stop moving before going to bed!"))
 			return false