docs/181134: Fix example for boot0cfg utility

[Vadim]

>Number:         181134
>Category:       docs
>Synopsis:       Fix example for boot0cfg utility
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-doc
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Thu Aug 08 07:40:02 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Vadim
>Release:        8.4
>Organization:
>Environment:
>Description:
Documentation at:
http://www.freebsd.org/cgi/man.cgi?query=boot0cfg&sektion=8

Section: EXAMPLES contains example of getting back to non-interactive booting by 
fdisk -B ad0

But this example will not work without setting kern.geom.debugflags=16



>How-To-Repeat:

>Fix:
Please, add extra lines to this example. Like:

-------------------------------------------
sysctl kern.geom.debugflags=16
fdisk -B ad0
sysctl kern.geom.debugflags=0
-------------------------------------------

Probably it will save a lot of time for people searching why fdisk -B ad0 not workig ....

>Release-Note:
>Audit-Trail:
>Unformatted: