doc/go1.15: add release notes for strconv

Updates #37419

Change-Id: Ic72bf0da914fa8a56570750b8fd4b4d09d2ed075
Reviewed-on: https://go-review.googlesource.com/c/go/+/236157
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: julieqiu

doc/go1.15.html

@@ -574,7 +574,13 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="strconv"><dt><a href="/pkg/strconv/">strconv</a></dt>
   <dd>
     <p><!-- CL 216617 -->
-      TODO: <a href="https://golang.org/cl/216617">https://golang.org/cl/216617</a>: add ParseComplex and FormatComplex
+      <a href="/pkg/strconv/#FormatComplex"><code>FormatComplex</code></a> and <a href="/pkg/strconv/#ParseComplex"><code>ParseComplex</code></a> are added for working with complex numbers.
+    </p>
+    <p>
+      <a href="/pkg/strconv/#FormatComplex"><code>FormatComplex</code></a> converts a complex number into a string of the form (a+bi), where a and b are the real and imaginary parts.
+    </p>
+    <p>
+      <a href="/pkg/strconv/#ParseComplex"><code>ParseComplex</code></a> converts a string into a complex number of a specificed precision. <code>ParseComplex</code> accepts complex numbers in the format <code>N+Ni</code>.
     </p>
   </dd>
 </dl><!-- strconv -->