Document fetchGit.
Document fetchGit.

Fixes #1981.

dtzWill/nix,rycee/nix,layus/nix,dtzWill/nix,zimbatm/nix,dezgeg/nix,ehmry/nix,layus/nix,pikajude/nix,NixOS/nix,peti/nix,rycee/nix,layus/nix,dezgeg/nix,vcunat/nix,ehmry/nix,peti/nix,dtzWill/nix,peti/nix,dtzWill/nix,ehmry/nix,zimbatm/nix,NixOS/nix,vcunat/nix,layus/nix,dtzWill/nix,vcunat/nix,NixOS/nix,shlevy/nix,NixOS/nix,vcunat/nix,zimbatm/nix,zimbatm/nix,dezgeg/nix,NixOS/nix,rycee/nix,rycee/nix,ehmry/nix,shlevy/nix,shlevy/nix,dezgeg/nix,pikajude/nix,layus/nix,peti/nix,ehmry/nix,pikajude/nix,peti/nix,dezgeg/nix,shlevy/nix,rycee/nix,ehmry/nix,shlevy/nix