Add doc-alias documentation

src/attributes/doc-alias.md

@@ -0,0 +1,23 @@
+# `doc_alias`
+
+The tracking issue for this feature is: [#50146]
+
+[#50146]: https://github.com/rust-lang/rust/issues/50146
+
+------------------------
+
+You can add alias(es) to an item when using the `rustdoc` search through the
+`doc(alias)` attribute. Example:
+
+```rust,no_run
+#![feature(doc_alias)]
+
+#[doc(alias = "x")]
+#[doc(alias = "big")]
+pub struct BigX;
+```
+
+Then, when looking for it through the `rustdoc` search, if you enter "x" or
+"big", search will show the `BigX` struct first.
+
+Note that this feature is currently hidden behind the `feature(doc_alias)` gate.