Missing xmldocs for Microsoft.Extensions.Caching.Memory CacheExtensions

There's no xmldocs for MemoryCacheExtensions, 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

As can be seen in this GH issue, and from my own experience, people often get tripped up by this fact.

Thanks!

Inizia con src/Microsoft.Extensions.Caching.Abstractions/MemoryCacheExtensions.cs e confronta la relativa documentazione XML con le annotazioni dell'API ConcurrentDictionary AddOrUpdate collegate nell'issue. Documenta i membri di MemoryCacheExtensions, incluso il comportamento di rientranza delle factory di GetOrCreate e GetOrCreateAsync, in modo che il browser API generato e IntelliSense spieghino questa limitazione.