Are CancellationTokenSources triggered when invalidated from cache? Hope so!

Consider the following:

```
_cache.Get(CacheKeys.DependentCTS).Cancel();
```

Here, we're manually acquiring a cached `CancellationTokenSource` and calling `Cancel`. This will invalidate the cache for anyone using that token, eg people who have done this:

```
var cts = new CancellationTokenSource();
_cache.Set(CacheKeys.DependentCTS, cts);

using (var entry = _cache.CreateEntry(CacheKeys.Parent))
{
// expire this entry if the dependant entry expires.
entry.Value = DateTime.Now;
entry.RegisterPostEvictionCallback(DependentEvictionCallback, this);

_cache.Set(CacheKeys.Child,
DateTime.Now,
new CancellationChangeToken(cts.Token));
}

```

It's not clear from the docs whether, when the cached `CancellationTokenSource` is expired from the cache, whether that has the effect of triggering all the dependant cached entries to be invalidated too. For instance, if I'm only caching my `CancellationTokenSource` for 5 minutes, I'd love when it's being evacuated from the cache for all the dependents to go with it. But I don't know whether cache evacuation will trigger the `.Cancel()` on the token. It's certainly my hope that is the behaviour but I'd love the docs to be clear on that.

I suspect that isn't what happens as it's a bit "magical"

At the moment, the way I read it, it suggests that only calling `_cache.Get(CacheKeys.DependentCTS).Cancel();` will trigger the invalidation; ie it has to be explicitly triggered. If that is the case, then guidance on how to do that would improve these docs I think. My guess would be that this would be this would be done using `RegisterPostEvictionCallback` if that's the case?

PS great work on the docs generally - they're really good!

https://docs.microsoft.com/en-us/aspnet/core/performance/caching/memory?view=aspnetcore-2.2#cache-dependencies

---
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* ID: 13f5f8f9-18dd-9c12-9ed1-fcc6b44ba7fd
* Version Independent ID: 640b4f36-5886-7b69-1c95-cfc8574a02fd
* Content: [Cache in-memory in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/performance/caching/memory?view=aspnetcore-2.2#feedback)
* Content Source: [aspnetcore/performance/caching/memory.md](https://github.com/aspnet/AspNetCore.Docs/blob/master/aspnetcore/performance/caching/memory.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-performance**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**