doc/go1.15: document go/printer.StdFormat

For #37419
For #37453
For #37476

Change-Id: Ia032ec844773af421bc4217d5dd6e60996d8e91f
Reviewed-on: https://go-review.googlesource.com/c/go/+/237740
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: ianlancetaylor

doc/go1.15.html

@@ -633,6 +633,16 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- fmt -->
 
+<dl id="go/printer"><dt><a href="/pkg/go/printer/">go/printer</a></dt>
+  <dd>
+    <p><!-- CL 231461 -->
+      The new <a href="/pkg/go/printer/#Mode"><code>Mode</code></a>
+      value <a href="/pkg/go/printer/#StdFormat"><code>StdFormat</code></a>
+      directs the printer to apply standard formatting changes while
+      printing the output.
+  </dd>
+</dl><!-- go/printer -->
+
 <dl id="io/ioutil"><dt><a href="/pkg/io/ioutil/">io/ioutil</a></dt>
   <dd>
     <p><!-- CL 212597 -->