Explicit scheduling requirements for online votes (#579)

Looking in the process docs, I was unable to find specific wording on
our requirement that votes be explicit in the agenda for online
meetings.

The "In-person meeting consensus" section of consensus.md has the
following paragraph:

> For in-person meetings, champions are expected to list points for
  which they will seek consensus in the meeting agenda, and new
  consensus points can be added in-person as the discussion proceeds.

I propose that we add language to the "Online consensus" section
disallowing consensus votes that were not on the agenda 24 hours
before the meeting (our current practice) and I further propose that
we make an exception for general interest votes on pre-proposals. The
goal of this exception is to eliminate artificial process delays for
moving proposals to phase 1, given that the technical bar for that
move is so low.

process/consensus.md

@@ -55,7 +55,10 @@ It is critical that work progresses between in-person meetings: agreed-upon
 designs need to move forward, and new ideas need to reach some level of maturity
 before being discussed in-person. To that end, this group can reach consensus
 online, either on GitHub repositories under the WebAssembly organization or in
-official video calls. In the latter case, decisions are recorded in meeting
+official video calls. In the latter case, the consensus vote must be added to
+the agenda at least 24 hours before the video call is scheduled to begin, except
+in the case of general interest votes moving pre-proposals to phase 1, which can
+be added as the discussion proceeds. Consensus decisions are recorded in meeting
 notes and published just like in-person meeting notes are published.
 
 We introduce the following concepts to help the online decision process: