Background color for caution blocks in the Handbook

[Warren Block]

On Sat, 5 Mar 2011, Daniel Gerzo wrote:

> On 4.3.2011 23:28, Glen Barber wrote:
>> Hi all,
>> 
>> While looking through the new HAST documentation in the Handbook, it
>> occurred to me that we don't have any special highlighting for 'caution'
>> tags.
>> 
>> I think maybe it would be good to have the caution sections stand out a
>> bit, since those sections might contain the makings of a foot canon if
>> the user is not careful.
>
> Generally, I think it's a good idea; the only question is what color to 
> choose. Well, I don't really care that much, but make sure to not let this go 
> into a bikeshed discussion ;-)

Red is already used for something, notes I think.  Yellow for caution 
seems appropriate.