webadmin: Refine CIDR validation message

Added an explanation of the valid values inside a CIDR address.

Change-Id: I906eb4a2b59a02c4ca22747cb56e8dbf14165590
Bug-Url: https://bugzilla.redhat.com/1078183
Signed-off-by: Lior Vernia <lvernia@redhat.com>