Mention IPv6 support in IP restriction

The 'Restrict group access by IP address' feature supports
passing and understanding both IPv4 and IPv6 CIDR
notation address ranges

It felt like a good idea to call this out explicitly because
IPv4 ranges are a subset of IPv6 and will not work to
effectively cover the growing number of ISPs using
IPv6 addresses for their users.

doc/user/group/index.md

@@ -637,7 +637,7 @@ To restrict group access by IP address:
 
 1. Go to the group's **Settings > General** page.
 1. Expand the **Permissions and group features** section.
-1. In the **Allow access to the following IP addresses** field, enter IP address ranges in CIDR notation.
+1. In the **Allow access to the following IP addresses** field, enter IPv4 or IPv6 address ranges in CIDR notation.
 1. Select **Save changes**.
 
    ![Domain restriction by IP address](img/restrict-by-ip.gif)