add page to the Rustdoc Book about unstable features

src/doc/rustdoc/src/SUMMARY.md

@@ -5,3 +5,4 @@
 - [The `#[doc]` attribute](the-doc-attribute.md)
 - [Documentation tests](documentation-tests.md)
 - [Passes](passes.md)
+- [Unstable features](unstable-features.md)

src/doc/rustdoc/src/unstable-features.md

@@ -0,0 +1,10 @@
+# Unstable features
+
+Rustdoc is under active developement, and like the Rust compiler, some features are only available
+on the nightly releases. Some of these are new and need some more testing before they're able to get
+released to the world at large, and some of them are tied to features in the Rust compiler that are
+themselves unstable. Several features here require a matching `#![feature(...)]` attribute to
+enable, and thus are more fully documented in the [Unstable Book]. Those sections will link over
+there as necessary.
+
+[Unstable Book]: ../unstable-book/