Clarify prioritisation for acquisition, conversion and telemetry group

Why is this change being made?

In the cases where there is no work left in a milestone or an engineer is blocked, I believe that it's on the engineer to decide what to work on. There is always something to improve, to learn or experiment on and we can trust our engineers to take a good decision.

This happened recently where an engineer was blocked and while they have trusted their gut feeling by working on something important, I'd have liked to see this reflected in our handbook.

After a conversation with @jeromezng I've confirmed the behaviour and reflected this in the MR.

To discuss

I deliberately have chosen "when there is no more work or they are blocked" as a wording to have a first iteration. However, I don't feel fully comfortable since it does not reflect reality. To me, it would mean that we always aim to complete 100% of a milestone, although that is not true since we have a goal to complete 80%. This would result into career development related tasks etc. not being accounted for at all. WDYT?

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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