io::Error docs mention that error::Error downcasting is yet to be added.

[sfackler]

http://doc.rust-lang.org/stable/std/io/struct.Error.html#method.new

The methods it talks about now exist, but are currently unstable: http://doc.rust-lang.org/stable/std/error/trait.Error.html#method.is. Not sure if the docs should be updated now or after stabilization

cc @steveklabnik

[sfackler]

Downcasting is now implemented but unstable.