Rust unstable book: basic desc and example for concat_idents.

src/doc/unstable-book/src/concat-idents.md

@@ -6,5 +6,17 @@ The tracking issue for this feature is: [#29599]
 
 ------------------------
 
+The `concat_idents` feature adds a macro for concatenating multiple identifiers
+into one identifier.
 
+## Examples
 
+```rust
+#![feature(concat_idents)]
+
+fn main() {
+    fn foobar() -> u32 { 23 }
+    let f = concat_idents!(foo, bar);
+    assert_eq!(f(), 23);
+}
+```