In order to expose a convenient API, it's unfortunately necessary to
lean on Go's interface{} quite a bit: in reality we only accept a
handful of types at each call site, but it's impossible to express this
using the type system.

Prior to this commit (as well as the ParsePattern commit), I exposed all
of this type information in the form of an enormous comment on web.Mux,
however this was pretty gross. Instead, let's use "vanity" type aliases
for interface{} to provide documentation about which types are accepted
where. This will hopefully make the API documentation easier to skim,
but doesn't affect any existing uses of Goji.

This commit also clarifies a couple other parts of the documentation.