add docs for memory locking functions
add docs for memory locking functions

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