Missing xmldocs for Microsoft.Extensions.Caching.Memory CacheExtensions

There's no xmldocs for [MemoryCacheExtensions](https://github.com/aspnet/Caching/blob/master/src/Microsoft.Extensions.Caching.Abstractions/MemoryCacheExtensions.cs), which makes the online API browser docs and intellisense of little use.

Also, these docs should really include a mention that the `GetOrCreate`/`GetOrCreateAsync` factories is not blocked for re-entrancy unless the programmer implements that themselves (easiest way is to use `Lazy`)

For an example of good documentation of the same topic for a different class, check out the Remarks for [ConcurrentDictionary AddOrUpdate](https://docs.microsoft.com/en-us/dotnet/api/system.collections.concurrent.concurrentdictionary-2.addorupdate?view=net-5.0)

As can be seen in [this GH issue](https://github.com/dotnet/runtime/issues/36499), and from my own experience, people often get tripped up by this fact.

Thanks!