WSUG: Convert ``Protocols'',  ``Messages'', and ``How Wireshark Works''
to AsciiDoc.

``How Wireshark Works'' is disabled, so its final output hasn't been
checked.

Leave most of the content intact for now.

Change-Id: Ia78951faa3ffd9c6d2e23067dbfb954ab4042f60
Reviewed-on: https://code.wireshark.org/review/5210
Reviewed-by: Gerald Combs <gerald@wireshark.org>