Spring Document default constructor as fallback for multiple non-@Autowired constructors

As mentioned by @ElmehdiBenh in https://github.com/spring-projects/spring-framework/issues/23263#issuecomment-606966126, our documentation currently fails to point out that a default constructor (if present) will be used as the fallback constructor for autowiring if multiple constructors are present and none of them is annotated with @Autowired.

We should therefore make this clear in the documentation, both in the Javadoc for @Autowired and in the reference manual.