The basic example I have in mind is to offer pages like http://git-scm.com/docs/git-commit which do little more than duplicate the manual pages, but it would also make sense to simply mirror the content of arc help and fancy it up with a bit of CSS and automatically generated links. There are three reasons I make this suggestion:
1) When I tried to get Arcanist up and running, my first instinct was to Google it, second instinct was to open the tool and browse the man pages. Having these pages for Git generally makes it easier to find help online, and it surely minimizes duplication of effort writing documentation.
2) Displaying web help offers the ability to get stylish with colors and fonts and retweet buttons, making it easier to read in general.
3) If man pages are linked to articles in the user documentation section, they are nudged that much closer to the "edit" buttons for contributors. You could even even try sending people to the public wiki pages to get things rolling.