Add missing API docs

These were written without much knowledge of the actix-web internals!
Please review carefully!

src/error.rs

@@ -521,12 +521,16 @@ impl ResponseError for UriSegmentError {
 /// Errors which can occur when attempting to generate resource uri.
 #[derive(Fail, Debug, PartialEq)]
 pub enum UrlGenerationError {
+    /// Resource not found
     #[fail(display = "Resource not found")]
     ResourceNotFound,
+    /// Not all path pattern covered
     #[fail(display = "Not all path pattern covered")]
     NotEnoughElements,
+    /// Router is not available
     #[fail(display = "Router is not available")]
     RouterNotAvailable,
+    /// URL parse error
     #[fail(display = "{}", _0)]
     ParseError(#[cause] UrlParseError),
 }