Document connectedline parameter for chan_iax2

The connectedline parameter for a chan_iax2 peer was undocumented. This patch
documents the options in the sample configuration file.

(closes issue ASTERISK-21953)
Reported by: Birger "WIMPy" Harzenetter
........

Merged revisions 394886 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 394890 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394894 65c4cc65-6c06-0410-ace0-fbb531ad65f3