Description 1:
Clicking a window while holding down this modifier key 
will move the window (left click), resize the window (middle click),
or show the window menu (right click). The left and right operations
may be swapped using the "mouse_button_resize" key. Modifier is
expressed as "<Alt>" or "<Super>" for example.

Description 2:
Set this to true to resize with the right button and show a menu with
the middle button while holding down the key given in "mouse_button_modifier";
set it to false to make it work the opposite way around.

If description 2 is the correct one, description one should be "[...] The middle and right operations [...]