Document HTTP Service Client support

[philwebb]

We need to add documentation for the properties, customizer support and @HttpServiceClient.

[tharun694]

Hi i seen the issues and want to contribute to your project. If you allow me to work on documentation i will make changes good for other contributors . thank you.

[cvsnehankita]

Hi Team, I would like to contribute to these fixes. I would be happy to work on the documentation and make improvements that benefit other contributors. Thank you!

[philwebb]

Thanks for the offer both, but I think this one might be better handled by someone from the team since there's quite a bit to document.