This is out of date! Please use the Twine wiki instead.
Having investigated the Twine engine code, I've found that the official passage syntax documentation(*) is incomplete. Here, then, is what I believe to be a complete list of Twine markup syntax. Almost all of it is derived from TiddlyWiki, and as such, some parts are not necessarily relevant to the typical Twine author. Nevertheless, they remain available.

http://www.glorioustrainwrecks.com/files/Twine1.3.5.MarkupSyntax_0.html