Clarify the real nature of the RelayState parameter for SSO and SLO

In the most simple case, the RelayState may be used as a "returnUrl",
but I think it's important to underline that the RelayState does not
necessarily need to be a return URL. Indeed, the SAML 2.0 specification
clarifies that a limit of max 80 characters exists for it (at least
in the case of the HTTP-Redirect binding) and that a protection method
against tampering is suggested.
Therefore, a return URL in general would probably be a non-ideal use of
the RelayState parameter, so let's give the latter the relevance it
deserves.

Author: mauromol

README.md

@@ -433,10 +433,10 @@ The AuthNRequest will be sent signed or unsigned based on the security settings
 
 The IdP will then return the SAML Response to the user's client. The client is then forwarded to the Attribute Consumer Service of the SP with this information.
 
-We can set a 'returnTo' url parameter to the login function and that will be converted as a 'RelayState' parameter:
+We can set a 'RelayState' parameter containing a return url to the login function:
 ```
-String targetUrl = 'https://example.com';
-auth.login(returnTo=targetUrl)
+String returnUrl = 'https://example.com';
+auth.login(relayState=returnUrl)
 ```
 The login method can receive 6 more optional parameters:
 - *forceAuthn* When true the AuthNRequest will have the 'ForceAuthn' attribute set to 'true'
@@ -605,10 +605,10 @@ The Logout Request will be sent signed or unsigned based on the security setting
 
 The IdP will return the Logout Response through the user's client to the Single Logout Service of the SP.
 
-We can set a 'returnTo' url parameter to the logout function and that will be converted as a 'RelayState' parameter:
+We can set a 'RelayState' parameter containing a return url to the login function:
 ```
-String targetUrl = 'https://example.com';
-auth.logout(returnTo=targetUrl)
+String returnUrl = 'https://example.com';
+auth.logout(relayState=returnUrl)
 ```
 
 Also there are 7 optional parameters that can be set: