Use fail(cause) in the docs

Author: mitsuhiko

book/src/custom-fail.md

@@ -47,15 +47,15 @@ more complex type like `Error` or your custom Error/ErrorKind.
 
 When you have a dependency which returns a different error type, often you will
 be inclined to add it as a variant on your own error type. When you do that,
-you should tag the underlying error as the `#[cause]` of your error:
+you should tag the underlying error as the `#[fail(cause)]` of your error:
 
 ```rust
 #[derive(Fail, Debug)]
 pub enum MyError {
     #[fail(display = "Input was invalid UTF-8 at index {}", _0)]
     Utf8Error(usize),
     #[fail(display = "{}", _0)]
-    Io(#[cause] io::Error),
+    Io(#[fail(cause)] io::Error),
 }
 ```
 

book/src/derive-fail.md

@@ -148,7 +148,7 @@ Backtrace type.
 In contrast to `backtrace`, the cause cannot be determined by type name alone
 because it could be any type which implements `Fail`. For this reason, if your
 error has an underlying cause field, you need to annotate that field with
-the `#[cause]` attribute.
+the `#[fail(cause)]` attribute.
 
 This can be used in fields of enums as well as structs.
 
@@ -162,7 +162,7 @@ use std::io;
 #[derive(Fail, Debug)]
 #[fail(display = "An error occurred.")]
 struct MyError {
-    #[cause] io_error: io::Error,
+    #[fail(cause)] io_error: io::Error,
 }
 
 /// MyEnumError::cause will return a reference only if it is Variant2,
@@ -172,6 +172,6 @@ enum MyEnumError {
     #[fail(display = "An error occurred.")]
     Variant1,
     #[fail(display = "A different error occurred.")]
-    Variant2(#[cause] io::Error),
+    Variant2(#[fail(cause)] io::Error),
 }
 ```