SpringBoot Improve documentation of tracing dependencies and configuration properties

Document which property should contain the Zipkin URL.

Comment From: wilkinsona

@RobertRad Thank you very much for making your first contribution to Spring Boot. I didn't want to waste more of your time so I went with an amended version of your first suggestion in the end. I've mentioned the configuration properties for Wavefront and Zipkin in each section where they're relevant. There's a bit of repetition but I think that's unavoidable without a much bigger overhaul of the documentation.