Aligned definitions of Performance/Potential Matrix with slide deck and training material

Why is this change being made?

Aligned with the Issue here: https://gitlab.com/gitlab-com/people-group/peopleops-eng/assessment-tool/-/merge_requests/34#note_428766863

We want to create consistency with all the definitions for each block in the matrix

cc @gl-peoplepartners

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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