Add a standalone page to discuss type expressions

[hegemonic]

We should have a standalone page that explains what sorts of type expressions are supported. The @param and @type tag pages should link to that page.

When we create this page, we should make sure it covers all of the different kinds of Closure Compiler type expressions, including function types, which are not currently documented as being supported.

Also, Closure Compiler provides a website where you can validate your code, including things like type expressions. It'd be useful to link to that page from the type expressions page.

[hegemonic]

See also the details in #21.