doc/go1.15: add release notes for time

Updates #37419

Change-Id: I2018b55f335400070bfa3573adab9549a5bf6a1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/236158
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: julieqiu

doc/go1.15.html

@@ -662,7 +662,7 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
     </p>
 
     <p><!-- CL 227878 -->
-      TODO: <a href="https://golang.org/cl/227878">https://golang.org/cl/227878</a>: quote original value in errors returned by ParseDuration
+      When returning an error, <a href="/pkg/time/#ParseDuration"><code>ParseDuration</code></a> now quotes the original value.
     </p>
   </dd>
 </dl><!-- time -->