_content/doc/go1.17: clarify x509 rewrite

Behavior is unchanged for WebPKI certificates, there are at least some
other changes in other cases.

Fixes golang/go#50518

Change-Id: I8335cfd4cf3880418614e0ef22c594a32c17fdc4
Reviewed-on: https://go-review.googlesource.com/c/website/+/377774
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Trust: DO NOT USE <iant@google.com>

_content/doc/go1.17.html

@@ -724,7 +724,8 @@ func Foo() bool {
     <p><!-- CL 274234 -->
       <a href="/pkg/crypto/x509/#ParseCertificate"><code>ParseCertificate</code></a>
       has been rewritten, and now consumes ~70% fewer resources. The observable
-      behavior has not otherwise changed, except for error messages.
+      behavior when processng WebPKI certificates has not otherwise changed,
+      except for error messages.
     </p>
 
     <p><!-- CL 321190 -->