Rollup merge of #139149 - mejrs:itaf, r=ehuss

unstable book: document import_trait_associated_functions

Documents https://github.com/rust-lang/rust/issues/134691 which was implemented in https://github.com/rust-lang/rust/pull/134754

src/doc/unstable-book/src/language-features/import-trait-associated-functions.md

@@ -0,0 +1,22 @@
+# import_trait_associated_functions
+
+The tracking issue for this feature is: [#134691]
+
+[#134691]: https://github.com/rust-lang/rust/issues/134691
+
+------------------------
+
+This feature allows importing associated functions and constants from traits and then using them like regular items.
+
+```rust
+#![feature(import_trait_associated_functions)]
+
+use std::ops::Add::add;
+
+fn main() {
+    let numbers = vec![1, 2, 3, 4, 5, 6];
+    let sum = numbers.into_iter().reduce(add); // instead of `.reduce(Add:add)`
+
+    assert_eq!(sum, Some(21));
+}
+```