nixos/doc: fix some manpage references

nixos/lib/systemd-unit-options.nix

···
         `asDropin` creates a drop-in file named `overrides.conf`.
         Mainly needed to define instances for systemd template units (e.g. `[e-mail]`).
 
-        See also systemd.unit(1).
+        See also {manpage}`systemd.unit(5)`.
       '';
     };
 
···
 
         This option creates a `.wants` symlink in the given target that exists
         statelessly without the need for running `systemctl enable`.
-        The in systemd.unit(5) manpage described `[Install]` section however is
+        The `[Install]` section described in {manpage}`systemd.unit(5)` however is
         not supported because it is a stateful process that does not fit well
         into the NixOS design.
       '';

nixos/modules/services/backup/restic.nix

···
             OnCalendar = "daily";
           };
           description = lib.mdDoc ''
-            When to run the backup. See man systemd.timer for details.
+            When to run the backup. See {manpage}`systemd.timer(5)` for details.
           '';
           example = {
             OnCalendar = "00:05";

nixos/modules/services/networking/kresd.nix

···
       example = [ "53" ];
       description = lib.mdDoc ''
         What addresses and ports the server should listen on.
-        For detailed syntax see ListenStream in man systemd.socket.
+        For detailed syntax see ListenStream in {manpage}`systemd.socket(5)`.
       '';
     };
     listenTLS = mkOption {
···
       example = [ "198.51.100.1:853" "[2001:db8::1]:853" "853" ];
       description = lib.mdDoc ''
         Addresses and ports on which kresd should provide DNS over TLS (see RFC 7858).
-        For detailed syntax see ListenStream in man systemd.socket.
+        For detailed syntax see ListenStream in {manpage}`systemd.socket(5)`.
       '';
     };
     listenDoH = mkOption {
···
       example = [ "198.51.100.1:443" "[2001:db8::1]:443" "443" ];
       description = lib.mdDoc ''
         Addresses and ports on which kresd should provide DNS over HTTPS/2 (see RFC 8484).
-        For detailed syntax see ListenStream in man systemd.socket.
+        For detailed syntax see ListenStream in {manpage}`systemd.socket(5)`.
       '';
     };
     instances = mkOption {

nixos/modules/system/boot/systemd/user.nix

···
       type = types.lines;
       example = "DefaultCPUAccounting=yes";
       description = lib.mdDoc ''
-        Extra config options for systemd user instances. See man systemd-user.conf for
+        Extra config options for systemd user instances. See {manpage}`systemd-user.conf(5)` for
         available options.
       '';
     };