Improve documentation (add reference of HttpClientCustomizer)

[mgalindor]

Would be great if you could add a mention in the documentation about the existence of the class HttpClientCustomizer

Additional context I was looking a way to decorate the the httpclient and I couldn't find a clean way until I open the code ( HttpClientFactory) and there I found the customizer is a great feature and I think would be awesome let people know it through the documentation