Clarifications on the different models and reference to further docs.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@178733 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/res_snmp.conf.sample

 ;
 ; Configuration file for res_snmp
+;---------------------------------
 ;
+; Res_snmp can run as a subagent or standalone SNMP agent. The standalone snmp
+; agent is based on net-snmp and will read a configuration file called
+; asterisk.conf in the net-snmp configuration file path, starting with
+; /etc/snmp on many systems.
+;
+; If you use the subagent model, you need to enable agentx in snmpd.conf
+; Note that you can only run one Asterisk on the system in this case.
+;
+; Please read documentat in doc/snmp.txt to get more information about
+; snmp support in Asterisk
 
 [general]
 ; We run as a subagent per default -- to run as a full agent