Small cleanups all over the design document. (backport #662)

[mergify]

Description

I happened to be reading the design document here, and I noticed a few small things that could use improvements. So this PR is a mish-mash of small improvements to that document.

Is this user-facing behavior change?

No, only updates to the documentation.

Did you use Generative AI?

No.

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This is an automatic backport of pull request #662 done by Mergify.

[mergify]

Cherry-pick of 1f43622 has failed:

On branch mergify/bp/jazzy/pr-662
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	both modified:   docs/design.md

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