Document handle_alloc_error feature in Unstable Book

src/doc/unstable-book/src/language-features/alloc-error-handler.md

@@ -0,0 +1,20 @@
+# `alloc_error_handler`
+
+The tracking issue for this feature is: [#51540]
+
+[#51540]: https://github.com/rust-lang/rust/issues/51540
+
+------------------------
+
+This attribute is mandatory when using the alloc crate without the std crate. It is used when handling out of memory (OOM) allocation error, and is called
+by `alloc::alloc::handle_alloc_error`
+
+``` rust,ignore (partial-example)
+#![feature(alloc_error_handler)]
+#![no_std]
+
+#[alloc_error_handler]
+fn foo(_: core::alloc::Layout) -> ! {
+    // …
+}
+```