Review and revise System.Collections.Concurrent examples

## Review and revise System.Collections.Concurrent examples

A number of the examples that illustrate the types in System.Collections.Concurrent are unambiguously single-threaded and synchronous. This makes them rather pointless. They should be revised to be meaningful. The examples (at least for class topics) include:

- [ConcurrentBag\](https://docs.microsoft.com/dotnet/api/system.collections.concurrent.concurrentbag-1#examples)
- [ConcurrentDictionary\](https://docs.microsoft.com/dotnet/api/system.collections.concurrent.concurrentdictionary-2#examples)
- [ConcurrentQueue\](https://docs.microsoft.com/dotnet/api/system.collections.concurrent.concurrentqueue-1)
[ConcurrentStack\](https://docs.microsoft.com/dotnet/api/system.collections.concurrent.concurrentstack-1)
[IProducerConsumerCollection\](https://docs.microsoft.com/en-us/dotnet/api/system.collections.concurrent.iproducerconsumercollection-1)

Related to dotnet/docs#7741