[Security] Authenticator methods description

[smnandre]

At first i just wanted to reword this sentence i found missleading.. in the Custom Authenticator page.

If null is returned, the request continues like normal (i.e. the controller matching the login route is called).

I think it should be:

- (i.e. the controller matching the login route is called)
+ (i.e. the controller matching the current route is called)

Because it can be the login route for some Authenticators, but it's not for stateless requests, Header tokens, remember me...

I then realize the "if / if" was the reason I found things a bit unclear at first sight.

onAuthenticationSuccess(Request $request, ...)

    If the user is authenticated, (...)

    If ``null`` is returned, (...)

I read this as some sort of "if / else" ... but the first "if" englobes the whole paragraph (it's true again in the second one).

So i tried to rewrite a bit (using the docblocks from the AuthenticatorInterface as inspiration)

[javiereguiluz]

Simon, sorry we missed this contribution. It's merged now. Thanks!