Add docs for raw-dylib to unstable book

src/doc/unstable-book/src/language-features/raw-dylib.md

@@ -0,0 +1,34 @@
+# `raw_dylib`
+
+The tracking issue for this feature is: [#58713]
+
+[#58713]: https://github.com/rust-lang/rust/issues/58713
+
+------------------------
+
+The `raw_dylib` feature allows you to link against the implementations of functions in an `extern`
+block without, on Windows, linking against an import library.
+
+```rust,ignore (partial-example)
+#![feature(raw_dylib)]
+
+#[link(name="library", kind="raw-dylib")]
+extern {
+    fn extern_function(x: i32);
+}
+
+fn main() {
+    unsafe {
+        extern_function(14);
+    }
+}
+```
+
+## Limitations
+
+Currently, this feature is only supported on `-windows-msvc` targets.  Non-Windows platforms don't have import
+libraries, and an incompatibility between LLVM and the BFD linker means that it is not currently supported on
+`-windows-gnu` targets.
+
+On the `i686-pc-windows-msvc` target, this feature supports only the `cdecl`, `stdcall`, `system`, and `fastcall`
+calling conventions.