Change to show how to write a 'new style' dissector (that is, one that returns the...

Change to show how to write a 'new style' dissector (that is, one that returns the number of bytes it was able to dissect).  I think the Developer's Guide (docbook/) probably needs similar updates but I'm a bit shy about editing XML with good old vi; maybe if I can figure out if I can build the doc in the first place...  Also rewrap a few pararaphs that went past 80 columns.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@19989 f5534014-38df-0310-8fa8-9805f1628bb7