Added page for feature to unstable book.

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+# `self_in_typedefs`
+
+The tracking issue for this feature is: [#49303]
+
+[#49303]: https://github.com/rust-lang/rust/issues/49303
+
+------------------------
+
+The `self_in_typedefs` feature gate lets you use the special `Self` identifier
+in `struct`, `enum`, and `union` type definitions.
+
+A simple example is:
+
+```rust
+#![feature(self_in_typedefs)]
+
+enum List<T>
+where
+    Self: PartialOrd<Self> // can write `Self` instead of `List<T>`
+{
+    Nil,
+    Cons(T, Box<Self>) // likewise here
+}
+```