AbstractAuthenticationFilterConfigurer failureUrl Javadoc is incorrect

The failureUrl(String) Javadoc states that this method is a shortcut for invoking failureHandler(AuthenticationFailureHandler), which is incorrect.

The behavior of failureUrl changes based on whether a custom loginPage is configured, while the behavior of failureHandler stays the same in either case.

When a custom login page is configured

http
	.formLogin((formLogin) -> formLogin
		.loginPage("/custom-login")
		.failureUrl("/failure")
	);

In this case, developers are required to process the specified URL ("/failure") to generate an error page.

When a custom login page is not configured

http
	.formLogin((formLogin) -> formLogin
		.failureUrl("/failure")
	);

In this case, the framework will process the specified URL ("/failure") and generate the default error page, which is simply the default login page with an error message.
If a developer has created a mapping for "/failure", it will be ignored.

This applies to formLogin, oauth2Login and saml2Login.

Start at the AbstractAuthenticationFilterConfigurer.failureUrl(String) Javadoc and compare its statement with the two loginPage configurations described in the issue. Update the documentation to distinguish custom and default login pages and note that the behavior also applies to formLogin, oauth2Login, and saml2Login; done when the Javadoc accurately reflects these cases.