Using overlays in config.nix

I’ve noticed that I’m unable to use the result of overlays in config.nix. What is the order in which various nixpkgs configurations are loaded, and where is the relevant documentation?

How do I manage a declarative system specification that makes use of overlays?

nixpkgs.overlays seems to be the correct option.

Is just searched for “overlays” on the options page.

PS: Is there a way to link directly to a package or option on the respective pages?