Add information about the meaning of Router<S> to Router documentation

[wkordalski]

Motivation

The meaning of S in Router<S> can be easily misunderstood,
and the explanation is well-hidden in Router::with_state() documentation.

See:

• `into_make_service` implemented for `Router<(), B>` #1602
• .with_state() expects argument with type () #2083
• the trait `for<'a> tower_service::Service<IncomingStream<'a>>` is not implemented for `Router<S>` #2564
• What `S` in `Router<S>` means #3218
• No method named into_make_service found for struct Router<S> #1592
• Hard coding Router type breaks tower-http middleware #2412

Solution

Add proper information to Router<S> documentation, that is much more visible to the framework user than a section in Router::with_state(..) documentation.

[mladedav]

Thanks for the clarification.