Add blocking manual job to the list of causes

What does this MR do?

We got a Support ticket where the customer could not see the visualization in the MR diff. We focused solely on the format of the cobertura.xml as that is one of the possible causes in the Troubleshooting section. It was found later that the pipeline had a manual blocking job that caused this.

While this is mentioned in the Limits section, when troubleshooting an issue, it can be easily missed and would resolve the issue faster if we had it documented under Troubleshooting section.

Related issues

Support ticket - internal only

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