funcs/func_jitterbuffer: Tweak documentation

This patch merely reformats and cleans up a bit of the jitterbuffer
documentation for the wiki.
........

Merged revisions 420639 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@420640 65c4cc65-6c06-0410-ace0-fbb531ad65f3