Update documentation on build stages
Update documentation on build stages

Noticed earlier that we'd forgotten to update this.
mingaldrichgan/gatsby,0x80/gatsby,0x80/gatsby,chiedo/gatsby,fabrictech/gatsby,mickeyreiss/gatsby,fk/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,Khaledgarbaya/gatsby,fk/gatsby,fk/gatsby,Khaledgarbaya/gatsby,rothfels/gatsby,0x80/gatsby,okcoker/gatsby,okcoker/gatsby,chiedo/gatsby,mickeyreiss/gatsby,Khaledgarbaya/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,fabrictech/gatsby,okcoker/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,danielfarrell/gatsby,fabrictech/gatsby,rothfels/gatsby,mickeyreiss/gatsby,chiedo/gatsby,danielfarrell/gatsby,gatsbyjs/gatsby