New documentation section about caching;

Please review the new section about ldap-backend and it's caching function. @blizzz 
Hm, elaborate here. If you want to have a most up-to-date information from LDAP and do not care too much about frequent LDAP requests, then you chose a smaller value (10s), if you're OK with having changes on LDAP wait for up to one hour (which is probably fine in many cases), then this is totally correct.