Pretty sure insomniac and/or dark_mousy included that in their guides.
Shows up as:
A good way to learn things like this, if you can't find it in a guide, is quoting a post that uses it, and looking at the code they used.
Code:
[spoiler=Title Text]Thing you want spoilered[/spoiler]
Title Text (Click to View)
Or this more practical example (Click to View)