docs: add details for client login final step

This tweaks the documentation on the main module, in order to
add some details on the outcome of the client login final step.
In particular, it clarifies the result of `ClientLogin::finish()`
both on success and on errors and it adds some intra-crate links
to the relevant structures and fields.

src/lib.rs

@@ -390,11 +390,18 @@
 //!
 //! ### Client Login Finish
 //! In the third step of login, the client takes as input a
-//! [`CredentialResponse`] from the server. The client runs
-//! [`ClientLogin::finish`] and produces an output consisting of a
-//! [`CredentialFinalization`] to be sent to the server to complete the
-//! protocol, the `session_key` sequence of bytes which will match the server's
-//! session key upon a successful login.
+//! [`CredentialResponse`] from the server and runs [`ClientLogin::finish`]
+//! on it.
+//! If the authentication is successful, then the client obtains a
+//! [`ClientLoginFinishResult`]. Otherwise, on failure, the
+//! algorithm outputs an [`InvalidLoginError`](errors::ProtocolError::InvalidLoginError)
+//! error.
+//!
+//! The resulting [`ClientLoginFinishResult`] obtained by client in this step
+//! contains, among other things, a [`CredentialFinalization`] to be sent to the server
+//! to complete the protocol, and a
+//! [`session_key`](struct.ClientLoginFinishResult.html#structfield.session_key)
+//! which will match the server's session key upon a successful login.
 //! ```
 //! # use opaque_ke::{
 //! #   errors::ProtocolError,