revise stale documentation for MAX_3D_TEXTURE_WIDTH
revise stale documentation for MAX_3D_TEXTURE_WIDTH

The renderer inherited from volspy dropped the multiresolution pyramid
long ago. This parameter only hints at the texture size to help
determine the ray-cast iteration step pitch.

informatics-isi-edu/synspy,informatics-isi-edu/synspy,informatics-isi-edu/synspy