doc/go1.17: document changes to net/http package

Changes include:
* ReadRequest function now returns an error when a request has multiple
  Host headers.

For #44513.
Updates #46015.

Change-Id: I48ea7c5cee3f1d1a247035fd37191362a53d1f04
Reviewed-on: https://go-review.googlesource.com/c/go/+/317914
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Heschi Kreinick <heschi@google.com>

doc/go1.17.html

@@ -423,7 +423,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     </p>
 
     <p><!-- CL 308952 -->
-      TODO: <a href="https://golang.org/cl/308952">https://golang.org/cl/308952</a>: make ReadRequest return an error when requests have multiple Host headers
+      The <a href="/pkg/net/http/#ReadRequest"><code>ReadRequest</code></a> function
+      now returns an error when the request has multiple Host headers.
     </p>
   </dd>
 </dl><!-- net/http -->