Update section 2.6 to handle the web portal uri being optional

[Kyle Larose]

Section 2.6

   *  The User Equipment queries the API to learn of its state of
      captivity.  If captive, the User Equipment presents the portal
      user interface from the Web Portal Server to the user.

[we previously discussed this UE behavior as optional.  I don't mind
having the text be descriptive like this, since it's describing the
diagram, and the diagram is not binding on all UEs, but it seemed worth
noting just in case.]

We could probably preface this section with a comment indicating that it's covering the casewhere the API provides a web portal uri