Fix subject-verb disagreements in security model documentation

[CamWass]

Summary

This PR fixes a few cases where verbs didn't match the plurality of the nouns, e.g. "[plural noun] becomes X" or "[singular noun] need X".

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit e5f35e7.

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