Overwrites of current element

It's unclear in the documentation that the broadcast block only try to deliver the current element to all the receivers. From the formulation BroadcastBlock<T> ensures that the current element is broadcast to any linked targets before allowing the element to be overwritten, I assumed that no messages would be lost which is not the case if the receivers cannot receive the message at the time. Perhaps a clarification could be added here.

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#### Document Details

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* ID: 8628ce7c-45cb-07f8-b9d9-8ed8373bc42e
* Version Independent ID: 8b97663e-f888-cddf-efcc-1712e93999fd
* Content: [BroadcastBlock<T> Class (System.Threading.Tasks.Dataflow)](https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.dataflow.broadcastblock-1?view=netcore-2.2)
* Content Source: [xml/System.Threading.Tasks.Dataflow/BroadcastBlock`1.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Threading.Tasks.Dataflow/BroadcastBlock`1.xml)
* Product: **dotnet-api**
* GitHub Login: @rpetrusha
* Microsoft Alias: **ronpet**