[docs] document configuring server for multi-compiler

[kellymears]

The reproduction case provided in #1702 calls .serve, .watch and .proxy from within the context of a child instance.

Anything related to the development server should be called only within the context of the parent instance.

This isn't really documented, but it should be documented here.

Demo

This is incorrect:

export default async bud => {
  await bud.make(`app`, async app =>
    app
      .watch([`resources/views/**/*`, `app/**/*`])
      .proxy(`https://sage.test`)
      .serve(`http://0.0.0.0:3000`)
  )

  await bud.make(`editor`, async editor => editor)
}

This is correct:

export default async bud => {
  bud
    .watch([`resources/views/**/*`, `app/**/*`])
    .proxy(`https://sage.test`)
    .serve(`http://0.0.0.0:3000`);

  await bud.make(`app`, async app => app)
  await bud.make(`editor`, async editor => editor)
}