doc/go1.15: add release notes for fmt

Updates #37419

Change-Id: I344dd93ed7a75f88e7f937c80f5a6ad6c0327a07
Reviewed-on: https://go-review.googlesource.com/c/go/+/236417
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: shaqque

doc/go1.15.html

@@ -390,7 +390,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="fmt"><dt><a href="/pkg/fmt/">fmt</a></dt>
   <dd>
     <p><!-- CL 215001 -->
-      TODO: <a href="https://golang.org/cl/215001">https://golang.org/cl/215001</a>: do not remove trailing zeros for %g and %G with #(sharp) flag
+      The printing verbs <code>%#g</code> and <code>%#G</code> now preserve
+      trailing zeros for floating-point values.
     </p>
   </dd>
 </dl><!-- fmt -->