Document OBJ Writer Design Choices

[JustinPrivitera]

We ought to document things like

• we're using effectively 1D texture so 2nd coordinate is always zero and texture file is just 1 pixel wide
• we're padding texture to inhibit texture wrapping (this is definitely a color-table driven decision...normally, 3D content developers want textures wrapped)
• We're setting number of entries in texture by fiat (256) (again primarily driven by the use of texture for color mapping and not actual texturing)
• the writer options

and other things as well.

See #19084 for details.