Add unstable book entry

Author: zetanumbers

src/doc/unstable-book/src/language-features/link-arg-attribute.md

@@ -0,0 +1,21 @@
+# `link_arg_attribute`
+
+The tracking issue for this feature is: [#99427]
+
+------
+
+The `link_arg_attribute` feature allows passing arguments into the linker
+from inside of the source code. Order is preserved for link attributes as
+they were defined on a single extern block:
+
+```rust
+#![feature(doc_cfg)]
+
+#[link(kind = "link-arg", name = "--start-group")]
+#[link(kind = "static", name = "c")]
+#[link(kind = "static", name = "gcc")]
+#[link(kind = "link-arg", name = "--end-group")]
+extern "C" {}
+```
+
+[#99427]: https://github.com/rust-lang/rust/issues/99427