For the committer's guide, I'd omit this entire paragraph. It's relevance is low for a how-to section, and how we do trust will be something we need to work out in the coming weeks and/or months. I think this paragraph is awkwardly written and a bit unclear. I think those things that it will create a misperception we'll need to clean up later. Ideally, we'd instead point to a trusted source for a more detailed explanation about git's signing model.
That whole paragraph is verbatim from the commit introducing signed pushes. Security is not my thing, and while I understand the concepts, I could not figure out what to write and how to briefly explain signed pushes, so I simply copied it.
Perhaps make the above code specific to FreeBSD, so that it doens't clobber other project configurations in case one uses a different ID / key there.
The paragraph is out of place and not that useful in this context. It's confusing and doesn't communicate the differences in signing commits, tags etc. That entire discussion is available elsewhere and we should just include something like
To understand the scope of different ways to sign commits, tags, etc, please see <link here>
I can supply the link if need be.