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: http://svn.asterisk.org/svn/asterisk/trunk@420640 f38db490-d61c-443f-a65b-d21fe96a405b