Update javadoc of Configurer classes that apply sensible defaults to describe how they're typically used

[philwebb]

We considered deprecating them because I'm was not sure they were really necessary anymore. But they are designed for users to inject. We could update the javadoc to help folks understand the pattern.

[philwebb]

They are designed to be injected by users so that they can configure their own instances. We should keep them.

[bhargavsai007]

Hi @philwebb . I have 3 years of experience with Spring Boot. I would like to start contributing to this Open Source community. So I would like to work on this issue. Could you help me in getting started ?

Thanks