Relay Entrypoints Documentation

Hi!

Didn't find it as an issue, so I think it would be nice to have one for tracking "Relay Entrypoints API" documentation situation.

https://relay.dev/docs/en/a-guided-tour-of-relay#preloading-data-for-transitions-in-parallel-with-code-client-preloading
> In order to do so, we can use Relay EntryPoints, which are a set of APIs for efficiently loading both the code and data dependencies of any view in parallel. Check out our api reference for Entry Points:

It raises some interest on solving this particular parallelization problem. Maybe I didn't get it right, but I didn't find its documentation anywhere. Is this too experimental, and will be improved before releasing some examples?