jit and vmap best practices

[nstarman]

Just a suggestion to add a small section to the docs about best practices for quaxifying jitted and vmapped functions. Thanks!

[patrick-kidger]

At least with JIT, ths usual best-practice of putting JIT at the top level applies.

For vmap, I don't think it matters: put these in whichever order describes the operations you're trying to perform.

Does that sound reasonable to you?

(I agree it'd be worth emphasising this somewhere.)