Background color for caution blocks in the Handbook

[Warren Block]

On Sat, 5 Mar 2011, Glen Barber wrote:

> I was originally hesitant to use yellow to avoid causing visual pain
> when reading.  How about the colors I have here:
>
> http://www.glenbarber.us/FreeBSD/doc/en_US.ISO8859-1/books/handbook/disks-hast.html

That is yellow.  On my monitor, the red text could stand to be a little 
darker for contrast.  Otherwise, looks good.