Ok cool - I just made a branch with an updated help page and pushed it. However, I can't see how I turn that branch into a PR - there's no option for 'new pull request' or equivalent.
Perhaps that's because I don't have contributor status. Feels a bit chicken - egg right now but, once I've made my first commit, I'll write all the steps into the help page for others to understand.
FYI, the branch I pushed was called 'update/wiki_text'