updated transmit_silence option documentation in asterisk.conf

This patch updates the transmit_silence option to better document
why the option exists, and what it affects.  Thanks to russell
for providing the verbage for this update.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@240887 65c4cc65-6c06-0410-ace0-fbb531ad65f3