Add more_io_inner_methods feature to unstable book.

src/doc/unstable-book/src/SUMMARY.md

@@ -160,6 +160,7 @@
     - [linked_list_extras](library-features/linked-list-extras.md)
     - [lookup_host](library-features/lookup-host.md)
     - [manually_drop](library-features/manually-drop.md)
+    - [more_io_inner_methods](library-features/more-io-inner-methods.md)
     - [mpsc_select](library-features/mpsc-select.md)
     - [n16](library-features/n16.md)
     - [never_type_impls](library-features/never-type-impls.md)

src/doc/unstable-book/src/library-features/more-io-inner-methods.md

+# `more_io_inner_methods`
+
+The tracking issue for this feature is: None.
+
+------------------------
+
+This feature enables several internal accessor methods on structures in
+`std::io` including `Take::{get_ref, get_mut}` and `Chain::{into_inner, get_ref,
+get_mut}`.