Update docs for stateless session management in 6.x

Describe the bug
Spring Security uses RequestAttributeSecurityContextRepository instead of the NullSecurityContextRepository for Stateless sessions.

To Reproduce
Run the program with minimal configuration:

@Configuration
class SecurityConfig {
    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http, UserDetailsService userDetailsService) throws Exception {
        http.authorizeHttpRequests(requests -> requests
                .anyRequest().authenticated()
        );
        http.userDetailsService(userDetailsService);
        http.httpBasic(Customizer.withDefaults());
        http.sessionManagement(sessionConfig -> sessionConfig.sessionCreationPolicy(SessionCreationPolicy.STATELESS));
        http.csrf(csrfCustomizer -> csrfCustomizer.disable());
        return http.build();
    }

    @Bean
    UserDetailsService userDetailsService() {
        InMemoryUserDetailsManager inMemoryUserDetailsManager = new InMemoryUserDetailsManager();
        inMemoryUserDetailsManager.createUser(
                User.builder().username("john").password("pass").roles("USER").build()
        );
        return inMemoryUserDetailsManager;
    }
}

Expected behavior
According to the documentation, Spring should use NullSecurityContextRepository but RequestAttributeSecurityContextRepository is used instead

Sample
There is a test in below repository that checks SecurityContextRepository used by BasicAuthenticationFilter

https://github.com/slawekludwiczak/spring-security-basic-repository

PS. maybe I don't understand something, but it is a bit weird for me that for Stateless sessions there is SessionManagementFilter registered