Add error code documentation

src/librustc_error_codes/error_codes/E0706.md

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+ `async fn`s are not yet supported in Rust.
+
+Erroneous code example:
+
+```compile_fail,edition2018
+trait T {
+    // Neither case is currently supported.
+    async fn foo() {}
+    async fn bar(&self) {}
+}
+```
+
+`async fn`s normally return an `impl Future`, making the following two examples equivalent:
+
+```edition2018,ignore (example-of-desugaring-equivalence)
+async fn foo() -> User {
+    unimplemented!()
+}
+// The async fn above gets desugared as follows:
+fn foo(&self) -> impl Future<Output = User> + '_ {
+    unimplemented!()
+}
+```
+
+But when it comes to supporting this in traits, there are [a few implementation
+issues][async-is-hard], one of which is that returning `impl Trait` in traits is not supported,
+as it would require [Generic Associated Types] to be supported:
+
+```edition2018,ignore (example-of-desugaring-equivalence)
+impl MyDatabase {
+    async fn get_user(&self) -> User {
+        unimplemented!()
+    }
+}
+
+impl MyDatabase {
+    fn get_user(&self) -> impl Future<Output = User> + '_ {
+        unimplemented!()
+    }
+}
+```
+
+Until these issues are resolved, you can use the [`async-trait` crate], which allows you to use
+this feature by sidesteping the language feature issue by desugaring to "boxed futures"
+(`Pin<Box<dyn Future + Send + 'async>>`):
+
+```edition2018,ignore (example-of-desugaring-equivalence)
+#[async_trait]
+impl MyDatabase {
+    async fn get_user(&self) -> User {
+        unimplemented!()
+    }
+}
+
+// The annotated impl above gets desugared as follows:
+impl MyDatabase {
+    fn get_user<'async>(
+        &'async self,
+    ) -> Pin<Box<dyn std::future::Future<Output = User> + Send + 'async>>
+    where
+        Self: Sync + 'async,
+    {
+        unimplemented!()
+    }
+}
+```
+
+Note that using these trait methods will result in a heap allocation per-function-call. This is not
+a significant cost for the vast majority of applications, but should be considered when deciding
+whether to use this functionality in the public API of a low-level function that is expected to be
+called millions of times a second.
+
+You might be interested in visiting the [async book] for further information.
+
+[`async-trait` crate]: https://crates.io/crates/async-trait
+[async-is-hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
+[Generic Associated Types]: https://github.com/rust-lang/rust/issues/44265
+[async book]: https://rust-lang.github.io/async-book/07_workarounds/06_async_in_traits.html