Adopted Change Set Q: the optional :update_users action

This is google doc issue 5, about change Set Q: the optional :update_users action

This was agreed to be adopted as an optional action.

I've added the `:update_users` and `:cancel_update_users` actions and the `:updating_users` state to the API. Note that these are all optional to support. The text mentions that to let clients know they are supported, they should be added in the advertisement RSpec. (this is different from the google doc, which states that this should be in GetVersion)

(this also changes some unrelated markup, sorry if that is confusing, that should have been a seperate commit)