_content/doc/go1.23: document known issue for generic alias types

For golang/go#68526.
For golang/go#65614.

Change-Id: I260da94ffaae0d6583e6396b0a78b46dd298313c
Reviewed-on: https://go-review.googlesource.com/c/website/+/600936
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Bypass: Robert Griesemer <gri@google.com>

_content/doc/go1.23.md

@@ -35,7 +35,8 @@ For motivation see the 2022 ["range-over-func" discussion](/issue/56413).
 
 <!-- go.dev/issue/46477, CL 566856, CL 586955, CL 586956 -->
 Go 1.23 includes preview support for [generic type aliases](/issue/46477).
-Building the toolchain with `GOEXPERIMENT=aliastypeparams` enables this feature.
+Building the toolchain with `GOEXPERIMENT=aliastypeparams` enables this feature within a package.
+(Using generic alias types across package boundaries is not yet supported.)
 
 ## Tools {#tools}