Replace "apply patch" Differential advice with comprehensive instructions on "Download Raw Diff"
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Description

I'm removing the "Apply Patch: arc patch Dxxx" custom field from the UI. This is pretty much purely an onboarding hint.

In the future, we should rework "Download Raw Diff" to be more general, like "Download Changes" which gives you more options (git fetch, arc patch, download raw diff, etc). See also T1715.