doc/go1.16: mention net/http.Transport.GetProxyConnectHeader

For #40700
For #41048

Change-Id: Ida6bcaaf5edaa2bba9ba2b8e02ec9959481f8302
Reviewed-on: https://go-review.googlesource.com/c/go/+/285594
Trust: Ian Lance Taylor <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: ianlancetaylor

doc/go1.16.html

@@ -807,6 +807,19 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
       <code>HTTPS_PROXY</code> is unset.
     </p>
 
+    <p><!-- 259917 -->
+      The <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
+      type has a new field
+      <a href="/pkg/net/http/#Transport.GetProxyConnectHeader"><code>GetProxyConnectHeader</code></a>
+      which may be set to a function that returns headers to send to a
+      proxy during a <code>CONNECT</code> request.
+      In effect <code>GetProxyConnectHeader</code> is a dynamic
+      version of the existing field
+      <a href="/pkg/net/http/#Transport.ProxyConnectHeader"><code>ProxyConnectHeader</code></a>;
+      if <code>GetProxyConnectHeader</code> is not <code>nil</code>,
+      then <code>ProxyConnectHeader</code> is ignored.
+    </p>
+
     <p><!-- CL 243939 -->
       The new <a href="/pkg/net/http/#FS"><code>http.FS</code></a>
       function converts an <a href="/pkg/io/fs/#FS"><code>fs.FS</code></a>