Improve Analysis documentation, to explain how to get failure

[RealAnna]

If an objective like the following fails, the overall status will not reflect that since the objective is not a key objective

objectives:
  - analysisValueTemplateRef:
      name: request-duration
    keyObjective: false
    target:
      failure:
        greaterThan:
          fixedValue: 3
      warning:
        greaterThan:
          fixedValue: 2
    weight: 1
  totalScore:
    passPercentage: 90
    warningPercentage: 75

However the status is neither explicitly set to pass so it is omitted...

DoD

• [ ] change the controller to always explicitly set pass true if we want to keep the behaviour consistent
• [ ] make sure to document this better in the Analysis ref and in the guide: keyObjective failure implies the entire analysis fails, if no key objective fails the analysis will pass.
• [ ] Extract some basic concept from https://medium.com/keptn/implementing-sli-slo-based-continuous-delivery-quality-gates-using-prometheus-9e17ec18ca36 to help people new to SLO/SLI