Document "smart group cache timeout" setting.

From #397

Thanks @eileenmcnaughton. Very useful.

As @michaelmcandrew asks here https://civicrm.stackexchange.com/q/31303/104 maybe it would be missing to document Administer > Customize Data and Screens > Search Preferences 'Smart group cache timeout' setting. I personally continue without being completely clear what is its purpose.