nir/lower_double_ops: add note for lowering mod

Juan A. Suárez requested to merge jasuarez/mesa:review/clarify-lower-mod into master

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.

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