unistd: doc updates based on review comments
unistd: doc updates based on review comments

These make the language consistent in the 3rd person, fix a few
minor mistakes, and remove some superflous prose.

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

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