The short answer, of course, is "check out individual open source projects" - I'm specifically wondering if you know of any projects (or centralized listings of projects, although I've searched and haven't found much) asking for help with documentation, diagramming, and generally making it easier for contributors to understand the codebase. Thanks in advance.
I would take a look at GitHub's open issues for things titled "add documentation" or similar:<p><a href="https://github.com/search?q=add+documentation&state=open&type=Issues" rel="nofollow">https://github.com/search?q=add+documentation&state=open&typ...</a>