Use exchange rather than operation to refer to a request and response

[wilkinsona]

When REST Docs began there was no term in the Spring portfolio describing a request being sent to an endpoint and a response being received so "operation" was used. Since then, "exchange" has been adopted in Spring Framework. I'd like REST Docs to switch to using "exchange", primarily because I think consistency across the two projects is important but also because I think exchange is a better term anyway.

To ease migration, I'd like to introduce the use of exchange in 2.1 and deprecate the use of operation. I'm not sure how complex this will prove to be. There's quite a bit of code in the org.springframework.restdocs.operation and org.springframework.restdocs.operation.preprocess packages although not all of it is public or commonly used. There's also the operation Asciidoctor macro to consider.