allow_failure default value with rules is unclear

Summary

Note: This is possibly a "bug" in our documentation, and not per se in the product itself.

---

We currently document the following behaviour of allow_failure in doc/ci/yaml/index.md#L561-565:

The default value for `allow_failure` is:

- `true` for [manual jobs](../jobs/job_control.md#create-a-job-that-must-be-run-manually).
- `false` for manual jobs that also use [`rules`](#rules).
- `false` in all other cases.

false for manual jobs that also use rules.

It is not clear what the context is.
Is this referencing when: manual and allow_failure that is nested within a rules block, or
is this referencing when: manual and allow_failure in a job's top level block?

When configured in a job's top level, the default value for allow_failure is true for manual jobs, regardless of any rules

Steps to reproduce

stages:
  - a
  - b
  
job1:
  stage: a
  script:
    - this will fail
  when: manual
  rules:
    - exists:
        - .gitlab-ci.yml

job2:
  stage: b
  script:
    - echo "job 2"

https://gitlab.com/nelbacha/demo-default-value-for-allow_failure/-/pipelines/537302873

---

diff: allow_failure: false

stages:
  - a
  - b
  
job1:
  stage: a
  script:
    - this will fail
  when: manual
  rules:
    - exists:
        - .gitlab-ci.yml
  allow_failure: false

job2:
  stage: b
  script:
    - echo "job 2"

https://gitlab.com/nelbacha/demo-default-value-for-allow_failure/-/pipelines/537303991

Example Project

https://gitlab.com/nelbacha/demo-default-value-for-allow_failure

What is the current bug behavior?

Documentation states allow_failure default value is false, for manual jobs with rules

What is the expected correct behavior?

tbd

Relevant logs and/or screenshots

See steps to reproduce

Output of checks

This bug happens on GitLab.com

Possible fixes