Move MP2RAGE cropping comment from "model" description to "task" description

[jcohenadad]

Checklist

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• I've given this PR a concise, self-descriptive, and meaningful title
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Description

Originally, this cropping advice was added to the model description. But, this description isn't actually displayed anywhere. Instead, the advice should have been added to the task description, which this PR does.

Linked issues

Fixes #4064