doc/go1.21: document GOEXPERIMENT=loopvar and invite feedback

For #60078.

Change-Id: I92ab1fc2f30e6bbaabd8e3e1b8834e6ddff2414d
Reviewed-on: https://go-review.googlesource.com/c/go/+/500958
TryBot-Bypass: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>

doc/go1.21.html

@@ -145,10 +145,11 @@ <h2 id="language">Changes to the language</h2>
 
 <!-- https://go.dev/issue/57969 -->
 <p>
-  <!-- TODO(rsc): add GOEXPERIMENT=loopvar -->
+  Go 1.21 includes a preview of a language change we are considering for a future version of Go:
+  making for loop variables per-iteration instead of per-loop, to avoid accidental sharing bugs.
+  For details about how to try that language change, see <a href="https://go.dev/wiki/LoopvarExperiment">the LoopvarExperiment wiki page</a>.
 </p>
 
-
 <p>
   TODO: complete this section
 </p>