[Captive-portals] Slimming down signalling text

[Kyle Larose <kyle@agilicus.com>]

As suggested at the capport group meeting, I have taken a stab at slimming down the text related to signalling in the architecture draft.  I have submitted a pull request: https://github.com/capport-wg/architecture/pull/16

Please take a look at the pull request. Feel free to suggest anything else I should remove (or add!) to help simplify things.

Thanks!

Kyle