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/services/databases/postgresql.nix

@@ -40,7 +40,7 @@ in
 
     services.postgresql = {
 
-      enable = mkEnableOption "PostgreSQL Server";
+      enable = mkEnableOption (lib.mdDoc "PostgreSQL Server");
 
       package = mkOption {
         type = types.package;