services.postgresql: add identMap example

      identMap = mkOption {

        type = types.lines;

        default = "";

        example = literalExample ''

          map-name-0 system-username-0 database-username-0

          map-name-1 system-username-1 database-username-1

        '';

        description = lib.mdDoc ''

          Defines the mapping from system users to database users.

          The general form is:

          map-name system-username database-username

          See the [auth doc](https://postgresql.org/docs/current/auth-username-maps.html).

        '';

      };