semanage-node.8: describe netmask

Network Mask argument for `semanage node` accepts also the simple CIDR
mask format, so let's document it.

Signed-off-by: Topi Miettinen <toiwoton@gmail.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>

python/semanage/semanage-node.8

@@ -45,7 +45,7 @@ Extract customizable commands, for use within a transaction
 Remove all local customizations
 .TP
 .I   \-M NETMASK, \-\-netmask NETMASK
-Network Mask
+Network Mask, either in CIDR (/16) or address mask notation (255.255.0.0, ffff::)
 .TP
 .I   \-t TYPE, \-\-type TYPE
 SELinux type for the object