Document Subscription Demand in `BroadcastDispatcher`

[maennchen]

Follow up to dicussion via Slack with @josevalim

I'm not sure if that explanation is good. I think it could be more concise.

[josevalim]

I think this still has a race, it is just less likely. Instead, you most likely want to do this on init:

{:producer, state, demand: :accumulate}

And then once all process subscribe, you call GenStage.demand(producer, :forward). :)

[maennchen]

@josevalim I've updated to the new solution. This is also way more elegant than subscribing asynchronously.

In my use case, I now have a little job that runs at the end of the containing Supervisors children, which calls all producing stages to change the demand.

[josevalim]

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