Address memory caching topic issues

[guardrex]

Cache in-memory in ASP.NET Core
https://docs.microsoft.com/aspnet/core/performance/caching/memory
performance/caching/memory.md

From @user135711 at #5495 (comment) ...

On the in memory Caching sample, please add an explanation of how cache dependencies work. I've been trying to figure it out from the source code and debugger, and the Docs don't provide sufficient explanation:

    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));
    }

How does the child token above communicate to the parent token to evict and what is the purpose of disposing of the parent token? Is this a hack where the parent eviction only occurs because the already marked as expired parent gets evicted when the child cancellation triggers a remove expired entries search?

Second suggestion add to sample app a compatible version of this dependent cache entries construction because it makes more sense: aspnet/Caching#48

Additional

• The sample app is MVC, not Razor Pages.
• The topic is in need of a UE pass.