doc/go1.16: add time/tzdata release note for CL 261877

For #40700

Change-Id: I056cef20a5f071977d0ae589c7a50d5f69af3283
Reviewed-on: https://go-review.googlesource.com/c/go/+/273166
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: tklauser

Diff for: doc/go1.16.html

@@ -701,7 +701,9 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="time/tzdata"><dt><a href="/pkg/time/tzdata/">time/tzdata</a></dt>
   <dd>
     <p><!-- CL 261877 -->
-      TODO: <a href="https://golang.org/cl/261877">https://golang.org/cl/261877</a>: use slim tz data format
+      The slim timezone data format is now used for the time zone database in
+      <code>$GOROOT/lib/time/zoneinfo.zip</code> and the embedded copy in this
+      package. This reduces the size of the time zone database by about 350 KB.
     </p>
   </dd>
 </dl><!-- time/tzdata -->