[Ice] change to trickle section 8.2

[Peter Saint-Andre <[e-mail]>]

Based on the review from Nils, in version -13 of the trickle I-D we made
one substantive change to the first paragraph of section 8.2, including
addition of a MUST-level requirement.

OLD

   Once all candidate gathering is completed or expires for a specific
   media stream, the agent will generate an "end-of-candidates"
   indication for that stream and convey it to the remote agent via the
   signaling channel.  The exact form of the indication depends on the
   application protocol.

NEW

   Once all candidate gathering is completed or expires for an ICE
   session associated with a specific media stream, the agent will
   generate an "end-of-candidates" indication for that session and
   convey it to the remote agent via the signaling channel.  Although
   the exact form of the indication depends on the application protocol,
   the indication MUST specify the generation (ufrag/pwd combination) so
   that an agent can correlate the end-of-candidates indication with a
   particular ICE session.

I didn't want this to slip by without notice. Please comment on the list
if you have concerns.

Peter