update documentation to reflect the changes in the way configure detects net-snmp.

(closes issue #12067)
Reported by: juggie
Patches:
      12067_snmp_doc.patch uploaded by juggie (license 24)
Tested by: juggie


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@110911 65c4cc65-6c06-0410-ace0-fbb531ad65f3