Docs: need to document more clearly what default `ShardOptions` actually uses

[Aaronontheweb]

Please describe what you are trying to understand

Need a top-level XML-DOC comment that explains what new ShardOptions() (no customization) actually enforces by default - users should be aware when persistence is required.

Akka.Hosting/src/Akka.Cluster.Hosting/AkkaClusterHostingExtensions.cs

Lines 156 to 158 in d8973b3

	public sealed class ShardOptions
	{