Rewrite the bit about null vs. non-null "tree" dissector arguments a

Rewrite the bit about null vs. non-null "tree" dissector arguments a
bit, so as not to imply that there's some form of global "mode"
Wireshark is in when it passes a null or non-null pointer (there isn't),
and to explicitly note that there is *no* guarantee about the value of
"tree" on the first call to the dissector.  (I.e., please do not build a
mental model of how Wireshark works in that regard, and write your
dissector based on that mental model - you *will* be wrong.)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@31560 f5534014-38df-0310-8fa8-9805f1628bb7