Fix staking bug (#5170)

* fix staking bug

* add some guarantee note in SessionManager while it is not fixed in staking

* bumpd impl version

substrate/frame/session/src/lib.rs

@@ -156,6 +156,8 @@ pub trait SessionManager<ValidatorId> {
 	/// `new_index` is strictly greater than from previous call.
 	///
 	/// The first session start at index 0.
+	///
+	/// `new_session(session)` is guaranteed to be called before `end_session(session-1)`.
 	fn new_session(new_index: SessionIndex) -> Option<Vec<ValidatorId>>;
 	/// End the session.
 	///