Add documentation for #647 (#648)
Add documentation for #647 (#648)

Pull request #647 introduced the matcher `shouldBeOneOf`. This matcher is going to be introduced in a future version of KotlinTest, and with it, the documentation must be current.

This wasn't added as part of #647 because we didn't want documentation to include a matcher not existent yet.

Part of #616
kotlintest/kotlintest,kotlintest/kotlintest,sksamuel/ktest,kotlintest/kotlintest