Document how to use `failwhen` to revert condition expected result

[olblak]

Summary

While answering this updatecli/updatecli#1137
I noticed that it's not obvious how to revert condition result

Basic example

conditions:
  jenkinsexist:
    name: Expect condition to succeed as Jenkins version exist
    kind: jenkins
    spec:
      version: '2.235'
      release: weekly
    failwhen: false
  jenkindonotexist:
    name: Expect condition to fail due to failwhen even if Jenkins version exist
    kind: jenkins
    spec:
      version: '2.235'
      release: weekly
    failwhen: true
  wrongjenkinsexist:
    name: Expect condition to fail as Jenkins version do not exist
    kind: jenkins
    spec:
      version: '2.999'
      release: weekly
    failwhen: false
  wrongjenkindonotexist:
    name: Expect condition to success due to failwhen even if Jenkins version do not exist
    kind: jenkins
    spec:
      version: '2.999'
      release: weekly
    failwhen: true

Motivation

Why are we doing this? What use cases does it support? What is the expected outcome?