Document what happens when DSL is invoked twice

[rwinch]

We should document in Javadoc and the reference what happens if the DSL is invoked twice. For example, what happens if the following is done:

http.authorizeHttpRequests(request -> request.anyRequest().permitAll());

http.authorizeHttpRequests(request -> request.anyRequest().denyAll())

Does this override values or does it add to them? There are other configurations where values are set vs added that would be interesting to explicitly document. For example

http.csrf().csrfTokenRepository(first); // ...
http.csrf().csrfTokenRepository(second); // ...

We should also document lambdas