docs: wrong description of test-stacktrace

[ericdallo]

I'm implementing support for test-stacktrace on https://github.com/afucher/clojure-repl-intellij and checking how cider does things, and noticed it uses that op, but the doc looks wrong.

[bbatsov]

My guess is that someone copy-pasted the docs from another op and didn't edit them fully. We'll have this fixed.