nir/lower_double_ops: add note for lowering mod

nir/lower_double_ops: add note for lowering mod

Add a note to clarify that while Vulkan allows mod(x,y) to be in [0, y]
range, OpenGL does not allow it, so the lowering ensures the result is
always in [0, y) range, as this lowering is shared by the Vulkan and
OpenGL implementation.

Reviewed-by: Elie Tournier <elie.tournier@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3315>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3315>