doc/overlays.xml: fix missing documentation for blas/lapack
doc/overlays.xml: fix missing documentation for blas/lapack

This adds some more information to the documentation as well as
addressing review from https://github.com/NixOS/nixpkgs/pull/85636.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs