So apparently, if you type a slash in the chat edit box, and then press tab, it will try to auto complete the slash command for you. For example, if you type /a and then press tab, if will suggest /afk. If you press tab again, it will suggest more commands beginning with a. If you just type a slash, you can tab through all available slash commands.
This also works with custom commands created by addons, of course. It doesn't work if the edit box is currently in whisper mode though, it has to be a regular channel like guild or say.
This might be common knowledge, but it's the first time I've seen this
