WSUG: Convert ``Telephony'' to AsciiDoc.

Leave most of the content intact for now.

Change-Id: I21aad681194d1bb1841e29f4ac41be4677fcb909
Reviewed-on: https://code.wireshark.org/review/3939
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>