permissions: docs - note about netmask value 0 in the database column

src/modules/permissions/doc/permissions_admin.xml

@@ -548,8 +548,9 @@ modparam("permissions", "ip_addr_col", "ip_address")
 		<title><varname>mask_col</varname> (string)</title>
 		<para>
 		Name of address table column containing the network mask of
-		the address.  Possible values are 0-32 for IPv4 and 0-128 for
-		IPv6 addresses.
+		the address. Possible values are 0-32 for IPv4 and 0-128 for
+		IPv6 addresses. If the value is 0, the network mask is set to 32
+		for IPv4 addresses and to 128 for IPv6 addresses.
 		</para>
 		<para>
 		<emphasis>