unistd: add documentation for setpgid
unistd: add documentation for setpgid

Signed-off-by: Paul Osborne <b6afffd6c1a00645fdd04b09edf63d034a949a39@gmail.com>

carllerche/nix-rust,geofft/nix-rust,MarkusJais/nix-rust,xd009642/nix,kamalmarhubi/nix-rust,Susurrus/nix,posborne/rust-nix,berkowski/nix,berkowski/nix,MarkusJais/nix-rust,kamalmarhubi/nix-rust,posborne/rust-nix,berkowski/nix,geofft/nix-rust,MarkusJais/nix-rust,carllerche/nix-rust,geofft/nix-rust,nix-rust/nix,carllerche/nix-rust,posborne/rust-nix,asomers/nix,kamalmarhubi/nix-rust,nix-rust/nix,xd009642/nix,xd009642/nix,Susurrus/nix,Susurrus/nix,asomers/nix