Closing issue 160

Issue 160 has been addressed with the following proposal:
  Remove the text in question and keep section 4 primarily
  the infoset description of the envelope with a reference
  to section 2 for the specifics of how to use the "actor"
  attribute.

-Dug

[1] http://www.w3.org/2000/xp/Group/xmlp-issues.html#x160