Potential update to the cron section of the handbook

[Bill Moran]

This change was provoked by the following thread:
http://docs.freebsd.org/cgi/mid.cgi?20040520205749.4392.qmail

I've tried to add enough warnings and other information to make
the difference clear without going overboard.

See rendered version here:
http://www.potentialtech.com/wmoran/configtuning-cron.html

If it looks good, I'll make a diff and file a PR.  Any
suggestions, complains, etc?

-- 
Bill Moran
Potential Technologies
http://www.potentialtech.com