Convert the Lua reference to AsciiDoc.

Move it to the Developer's Guide while we're here.

Nudge the markup in epan/wslua where needed. Note that we should
probably convert it to AsciiDoc (if we're going to keep it in the DG) or
Doxygen.

Change-Id: Ie175111043f98b7a37eeeb8d185a833d8e866f8b
Reviewed-on: https://code.wireshark.org/review/5203
Reviewed-by: Gerald Combs <gerald@wireshark.org>