Document multi-project config options

There is some decent [documentation for the single-project config](https://github.com/facebook/relay/tree/main/packages/relay-compiler#configuration) options, but no mention of multi-project configs in any of the documentation. The [test project](https://github.com/facebook/relay/blob/d6ee691846bd806a113922a466dbb4d7b6b562fc/compiler/test-project/relay.json) uses a config in the multi-project format and [the source for the format](https://github.com/facebook/relay/blob/d6ee691846bd806a113922a466dbb4d7b6b562fc/compiler/crates/relay-compiler/src/config.rs#L523) is pretty easy to read, but it'd be nice to have this documented in the same place as the single-project format.