Change ACLs to apply on proposed/* branches

Change ACLs so that they would apply to proposed/* branches instead of
milestone-proposed branches, and adjust documentation to match.

NB: The top-level project.config should also be changed so that its ACLs
apply to proposed/* branches. It's apparently not driven by the config
repository.

Change-Id: Iad6991315b1d526026e0aeb0e968ed85dd317fac
Merge "Change ACLs to apply on proposed/* branches"