Describe the new topic

I had to look inside the source code to understand how SignIn and SignOut flows work, and which party needs to implement the logic. (Docs do not mention IAuthenticationSignInHandler). Describing what SignIn and SignOut are intended for would be useful.

Also, I could not find a place where docs talk about IAuthenticationRequestHandler and what role it plays in authentication. Especially for RemoteAuthenticationHandler implementations which expect callback from remote identity providers.

IMO, this is an important subject related to authentication which should be added to the documentation.