git-maintenance: More details in the documentation

Signed-off-by: Sumner Evans <me@sumnerevans.com>

modules/programs/git.nix

@@ -222,6 +222,9 @@ in {
           description = ''
             Enable the automatic {command}`git maintenance`.
 
+            If you have SSH remotes, set {option}`programs.git.package` to a
+            git version with SSH support (eg: `pkgs.gitFull`).
+
             See <https://git-scm.com/docs/git-maintenance>.
           '';
         };