Add rule about meaning of inline with async

On lang, we had earlier made a decision about how `inline` should
apply to `async fn`.  Let's document that here.

Author: traviscross

src/attributes/codegen.md

@@ -74,6 +74,12 @@ The `inline` attribute supports these modes:
 r[attributes.codegen.inline.trait]
 When `inline` is applied to a function in a [trait], it applies only to the code of the [default definition].
 
+r[attributes.codegen.inline.async]
+When `inline` is applied to an async function or async closure, it applies only to the code of the generated `poll` function.
+
+> [!NOTE]
+> For more details, see [Rust issue #129347](https://github.com/rust-lang/rust/issues/129347).
+
 r[attributes.codegen.inline.externally-exported]
 The `inline` attribute is ignored if the function is externally exported with [`no_mangle`] or [`export_name`].