Add doc for all generated items

[Yamakaky]

No description provided.

[Yamakaky]

Closing since you can always do

error_chain! {
  links {
    inner::Error, Inner, #[doc = "Inner"];
  }
  errors {
    /// A variant
    Variant
  }
}

See https://stackoverflow.com/questions/33999341/generating-documentation-in-macros/33999625#33999625.
See #63 for a better syntax.