Update supply chain semgrepignore docs

[mmcqd]

The behavior described in the docs has been different for a while, it's simpler now.

Please ensure

• A subject matter expert (SME) reviews the content
• A technical writer reviews the content or PR
• This change has no security implications or else you have pinged the security team
• Redirects are added if the PR changes page URLs
• If you have changed any header tag links (doc/#this-kind-of-anchor), update all instances of that link

[netlify]

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[s-santillan]

@mmcqd Thanks! Gonna go ahead and merge this now as it is an S-sized PR. We thank you for your service.