doc: Consolidate NetRestrict format docmentation

We were specifying exactly the same format in two different places;
consolidate them into one place. In addition, explicitly say there are
is no way to specify a range of addresses, in case some people are
confused by the previous versions of this man page that erroneously
said you could use 255 as a wildcard.

Reviewed-on: http://gerrit.openafs.org/7565
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit d4bab02f023f53041c3ac58fd75cffd27ec92df0)

Change-Id: Ie138f8b6e3070d0bc92adae7dae90a26cbd78001
Reviewed-on: http://gerrit.openafs.org/7590
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>