doc/go1.20: resolve root bundle package TODO

We're unlikely to get this package out of the door all that soon. For
now add a note that SetFallbackRoots will be most commonly used with
an TBA package, and link the tracking issue.

We could also just remove the "It will most commonly be used ..."
sentence.

Change-Id: Ie96134d757f5b4c69f1878d53c92b5ed602671e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/454056
Reviewed-by: Julie Qiu <julieqiu@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Roland Shoemaker <roland@golang.org>

doc/go1.20.html

@@ -638,7 +638,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       The new <a href="/pkg/crypto/x509/#SetFallbackRoots"><code>SetFallbackRoots</code></a>
       function allows a program to define a set of fallback root certificates in case the
       operating system verifier or standard platform root bundle is unavailable at runtime.
-      It will most commonly be used with the new TODO LINK TO x/crypto PACKAGE ONCE IT EXISTS.
+      It will most commonly be used with a new package, <a href="/pkg/golang.org/x/crypto/x509roots/fallback">golang.org/x/crypto/x509roots/fallback</a>,
+      which will provide an up to date root bundle.
     </p>
   </dd>
 </dl><!-- crypto/x509 -->