docs: CI rules: manual does not set 'allowed to fail' automatically

What does this MR do?

I looked into why rules:manual might be causing pipelines to block (internal: slack thread)

Quoting from a code review:

NOTE: Unlike when: manual, rules:manual is implying allow_failure: false by default. We need to explicitly pass allow_failure: true in this case.

Set of tests on gitlab.com:

• old when:manual behaviour
• implicit rules: behaviour
• fixed rules: behaviour

I've added a section for documenting how rules differs from previous behaviour, and noted it in places where manual jobs were documented as non-blocking.

Related issues

#34077 (closed)

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