Clarify Sleuth support

It's not at all clear from the documentation what is traced in the presence of Spring Cloud Sleuth. Are tracing headers added to all client-server calls? How's that helpful if the server doesn't return those back? Please update the docs to clarify, preferably using an image like [this](https://raw.githubusercontent.com/spring-cloud/spring-cloud-sleuth/2.2.x/docs/src/main/asciidoc/images/trace-id.png)

Review the existing documentation about Spring Cloud Sleuth tracing and compare it with the linked trace-id image. Clarify which client-server calls and headers are traced, including whether servers return them, and include an explanatory image; done when the documentation answers the questions raised in the issue.

Written by the indexing model from the issue text.