Clarify plagiarism doc
Clarify plagiarism doc

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Test plan
- N/A

Change-Id: I9b61dc6a82ddb380a5ebda31c4e15cad5b30562a
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/292042
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Evan Battaglia <e557ffdfc4b80aab44858030b0ae5370b86cb7b1@instructure.com>
QA-Review: Mysti Lilla <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>
Product-Review: Mysti Lilla <431d062aff1e8f25416b3a7b732fe6effdb03f9a@instructure.com>

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