docs/nir: Document all the ALU opcodes

As with ISL, I'd like to start documenting NIR for realz. Along those lines, this starts off the party by providing docs for every NIR ALU opcode. It's not much but it does provide a bit of info for each op.

Easiest way to see the output is to look at the build artifacts from this MR:

https://mesa.pages.freedesktop.org/-/mesa/-/jobs/11031726/artifacts/public/nir/alu.html