nixos/snapper: improve config example

In the default configuration we have timers for creating and deleting
snapper snapshots, and it looks like if we just create configs with
correct mountpoints we will get automatic snapshots (which either
used to be true, or seems to be only true on Archlinux according to
their wiki). In default snapper configuration TIMELINE_CREATE and
TIMELINE_CLEANUP are set to "no", so just providing configs won't
be enough for having automatic backups, which are the main usecase
for snapper. In other linux distributions you would use `snapper
create-config` to generate configs for partitions and you'd have a
chance to notice that TIMELINE_CREATE is set to no. Also, my guess is
that it might be set to no by default for safety reasons in regular distros,
so that the config won't be actioned upon until the user finishes
customizing it.

nixos/modules/services/misc/snapper.nix

@@ -48,6 +48,8 @@ in
           subvolume = "/home";
           extraConfig = ''
             ALLOW_USERS="alice"
+            TIMELINE_CREATE=yes
+            TIMELINE_CLEANUP=yes
           '';
         };
       };