Update the unstable book

Author: Zoxc

Diff for: src/doc/unstable-book/src/language-features/rustc-private.md

@@ -0,0 +1,11 @@
+# `rustc_private`
+
+The tracking issue for this feature is: [#27812]
+
+[#27812]: https://github.com/rust-lang/rust/issues/27812
+
+------------------------
+
+This feature allows access to unstable internal compiler crates.
+
+Additionally it changes the linking behavior of crates which have this feature enabled. It will prevent linking to a dylib if there's a static variant of it already statically linked into another dylib dependency. This is required to successfully link to `rustc_driver`.