Document Named Collectives

[lockwo]

Named collectives have come up before in documentation for pmap/shmap, but usually links to https://jax.readthedocs.io/en/latest/jax.lax.html#parallel-operators (other relevant issue: https://github.com/google/jax/issues/13794). However, these also work for things such as vmap and I think warrant more documentation, since I discovered them via: https://github.com/google/jax/discussions/22974#discussioncomment-10317061. My proposal is to have a list of all collectives that work under vmap (somewhere) in the docs.

[jakevdp]

It seems like this would best fit in an advanced vmap guide, probably related to the work in #18585 (which I've neglected for far too long...)

[doshi-kevin]

Are there any links left ?