Improve docs for relay.config.js with Typescript support from JSDoc

It would be good if this section:
https://relay.dev/docs/getting-started/installation-and-setup/#compiler-configuration

Had the same documenation like Next.js:
https://nextjs.org/docs/basic-features/typescript#type-checking-nextconfigjs

So the editor can understand the shape of the config.
https://www.typescriptlang.org/docs/handbook/jsdoc-supported-types.html#type