Add example to Chain iterator

src/error.rs

@@ -423,6 +423,24 @@ impl ErrorImpl<()> {
 }
 
 /// Iterator of a chain of source errors.
+///
+/// This type is the iterator returned by [`Error::chain`].
+///
+/// # Example
+///
+/// ```
+/// use anyhow::Error;
+/// use std::io;
+///
+/// pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
+///     for cause in error.chain() {
+///         if let Some(io_error) = cause.downcast_ref::<io::Error>() {
+///             return Some(io_error.kind());
+///         }
+///     }
+///     None
+/// }
+/// ```
 pub struct Chain<'a> {
     next: Option<&'a (dyn StdError + 'static)>,
 }