docs: update merge request coach docs
Why is this change being made?
Base doff this conversation: https://gitlab.slack.com/archives/C2T9APP9C/p1587034575017600, we want to extend the documentation to add the correct time period for mr coach's before reaching back out to community members.
We want to add this change to stop future mr coach's having the same question
Does this MR meet the acceptance criteria?
Conformity
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Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what