Improve documentation and test coverage for complicated request waits and captures

[xylix]

User use case from RF Slack: https://robotframework.slack.com/archives/C015KB1QSDN/p1608553440345200 , inlined here for archiving

"
I'm returning to the "intercept my app's Graphql responses" problem, except this time on behalf of a colleague who wants to intercept REST responses where (again) the request url strings are all the same, and the requests differ by what’s in the request body.
... so, I think we should be able to do this by Wait For Response with matcher= a JS function that looks for something in the request body. If the request I'm looking for is the one that has a body

{
"operationName":"sitesSummary",
"variables":{"startTime":1608548953678,"endTime":1608552553678},
"query": <... all the rest of it ...>
}

then I think something close to this ought to work:

${promise}=     Promise To            Wait For Response     matcher= (requestBody) => {requestBody.hasOwnProperty("operationName") && requestBody["operationName"] === "sitesSummary";}    timeout=10

It doesn’t – should it? Am I doing something wrong? (It might help if I knew more Javascript than a few hours of going through tutorials)
thanks for any advice! (edited)