doc/go1.17: document changes to os package

Documents the changes to File.WriteString method.

For #44513.
Fixes #46018.

Change-Id: I3a8ef9df9f84662614d54802710bd705d626b995
Reviewed-on: https://go-review.googlesource.com/c/go/+/317910
Reviewed-by: Dmitri Shuralyov <[email protected]>
Trust: Cherry Mui <[email protected]>

Author: VishalDalwadi

doc/go1.17.html

@@ -428,7 +428,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
   <dd>
     <p><!-- CL 268020 -->
-      TODO: <a href="https://golang.org/cl/268020">https://golang.org/cl/268020</a>: avoid allocation in File.WriteString
+      The <a href="/pkg/os/#File.WriteString"><code>File.WriteString</code></a> method
+      has been optimized to no longer make a copy of the input string.
     </p>
   </dd>
 </dl><!-- os -->