iPhoneDevWiki runs on the MediaWiki software, the same as Wikipedia and many other large wikis, so most help for editing Wikipedia/MediaWiki will apply here too.
Help with markup (page editing syntax)
See Wikipedia's formatting cheatsheet for how to do general wiki markup (italics, bold, links, images, and so on).
For help making tables, see MediaWiki's table formatting page.
How to make new articles (new pages)
An easy way to make a new article: type your desired article title into the search box, and click the "Create the page" link on the search results page. Then type in the content that you want, and save it.
Fancy syntax highlighting
You can make code snippets nicely syntax-highlighted using these tags:
<source lang=objc> NSObject *anObject = [[NSObject alloc] init]; </source>
Which will produce an output that looks like the following:
NSObject *anObject = [[NSObject alloc] init];
To add a helpful navigation box to the bottom of an article documenting a class, you can use tags like these:
If you add this navigation box to an article, the article link will automatically show up within that navigation box.
How to add categories to an article
To put an article in a category, add one or more lines like this to the bottom of the article:
[[Category:Category name]] [[Category:Another category name]]
If a category doesn't exist yet, it'll be automatically created when you add the category to an article.
A list of existing categories that you may want to choose from: Special:Categories.