nixos/*: automatically convert option descriptions

conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running

    nix-doc-munge nixos/**/*.nix
    nix-doc-munge --import nixos/**/*.nix

the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.

nixos/modules/programs/bash/bash-completion.nix

@@ -7,7 +7,7 @@ let
 in
 {
   options = {
-    programs.bash.enableCompletion = mkEnableOption "Bash completion for all interactive bash shells" // {
+    programs.bash.enableCompletion = mkEnableOption (lib.mdDoc "Bash completion for all interactive bash shells") // {
       default = true;
     };
   };