Add asm-goto to unstable book

Author: nbdd0121

src/doc/unstable-book/src/language-features/asm-goto.md

@@ -0,0 +1,24 @@
+# `asm_goto`
+
+The tracking issue for this feature is: [#119364]
+
+[#119364]: https://github.com/rust-lang/rust/issues/119364
+
+------------------------
+
+This feature adds a `label <block>` operand type to `asm!`.
+
+Example:
+```rust,ignore (partial-example, x86-only)
+
+unsafe {
+    asm!(
+        "jmp {}",
+        label {
+            println!("Jumped from asm!");
+        }
+    );
+}
+```
+
+The block must have unit type.