Add appendix for list of undocumented features

Re: rust-lang#9

src/SUMMARY.md

@@ -56,3 +56,5 @@
     - [Behavior not considered unsafe](behavior-not-considered-unsafe.md)
 
 [Appendix: Influences](influences.md)
+
+[Appendix: As-yet-undocumented Features](undocumented.md)

src/undocumented.md

@@ -0,0 +1,36 @@
+# As-yet-undocumented Features
+
+Several accepted, stabilized, and implemented RFCs lack documentation in this
+reference, The Book, _Rust by Example_, or some combination of those three.
+Until we have written reference documentation for these features, we provide
+links to other sources of information about them. Therefore, expect this list
+to shrink!
+
+- [`libstd` facade]
+- [Trait reform] – some partial documentation exists (the use of `Self`), but
+  not for everything: e.g. coherence and orphan rules.
+- [Attributes on `match` arms] – the underlying idea is documented in the
+  [Attributes] section, but the applicability to internal items is never
+  specified.
+- [Flexible target specification] - Some---but not all---flags are documented
+  in [Conditional compilation]
+- [Unambiguous function call syntax]
+- [Require parentheses for chained comparisons]
+- [Integer overflow not `unsafe`] - documented with a reference to the RFC, but
+  requires further details
+- [`dllimport`] - one element mentioned but not explained at [FFI attributes]
+- [define `crt_link`]
+- [define `unaligned_access`]
+
+[`libstd` facade]: https://github.com/rust-lang/rfcs/pull/40
+[Trait reform]: https://github.com/rust-lang/rfcs/pull/48
+[Attributes on `match` arms]: https://github.com/rust-lang/rfcs/pull/49
+[Flexible target specification]: https://github.com/rust-lang/rfcs/pull/131
+[Conditional compilation]: attributes.html#conditonal-compilation
+[Unambiguous function call syntax]: https://github.com/rust-lang/rfcs/pull/132
+[Require parentheses for chained comparisons]: https://github.com/rust-lang/rfcs/pull/558
+[Integer overflow not `unsafe`]: https://github.com/rust-lang/rfcs/pull/560
+[`dllimport`]: https://github.com/rust-lang/rfcs/pull/1717
+[FFI attributes]: attributes.html#ffi-attributes
+[define `crt_link`]: https://github.com/rust-lang/rfcs/pull/1721
+[define `unaligned_access`]: https://github.com/rust-lang/rfcs/pull/1725