Revise ioctl module documentation
Revise ioctl module documentation

This refactors the examples to more directly address the exact use cases
for the `ioctl!` macro that `nix` provides. Additionally other macros
that should not be used by end users are no longer discussed.

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