New review comment by ahesford on void-packages repository

https://github.com/void-linux/void-packages/pull/30552#discussion_r622457681

Comment:
I don't think the manual should call out poetry-core here. It is not the only PEP-517 builder and doesn't deserve special mention, especially because nothing in the repository currently uses it. To be somewhat generic, I'd recommend dropping this paragraph and adding a sentence at the end of the first paragraph (Line 1534) along the lines of

    The `python3-pep517` build style does not depend on a specific build backend, so packages will need to add an appropriate backend provider to `hostmakedepends`.

I don't think the dependency mapping content is important here; discussion of the `setuptools` mapping was included because `setuptools` has a nasty tendency of fetching and locally installing everything needed to build a package. For the `pep517` build style, we explicitly disable fetching to force hard errors when dependencies are improperly specified.