doc: add release notes for database/sql

Change-Id: I032a3342c3ac899a39d357420b981b6f281518f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/181939Reviewed-by: Daniel Theophanes <kardianos@gmail.com>

doc/go1.13.html

@@ -162,11 +162,11 @@ TODO
 <dl id="database/sql"><dt><a href="/pkg/database/sql/">database/sql</a></dt>
   <dd>
     <p><!-- CL 170699 -->
-      TODO: <a href="https://golang.org/cl/170699">https://golang.org/cl/170699</a>: add NullTime
+      The new <a href="/pkg/database/sql/#NullTime"><code>NullTime</code></a> type represents a <code>time.Time</code> that may be null.
     </p>
 
     <p><!-- CL 174178 -->
-      TODO: <a href="https://golang.org/cl/174178">https://golang.org/cl/174178</a>: add NullInt32
+      The new  <a href="/pkg/database/sql/#NullInt32"><code>NullInt32</code></a> type represents an <code>int32</code> that may be null.
     </p>
 
 </dl><!-- database/sql -->