doc/go1.17: document new go/build/BuildContext.ToolTags field

For #44513

Change-Id: Ib21af742e574fcaa7e38bb437d42dbeed9d01f0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/323350
Trust: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: ianlancetaylor

doc/go1.17.html

@@ -413,6 +413,17 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- flag -->
 
+<dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
+  <dd>
+    <p><!-- CL 310732 -->
+      The new
+      <a href="/pkg/go/build/#Context.ToolTags"><code>Context.ToolTags</code></a>
+      field holds the build tags appropriate to the current Go
+      toolchain configuration.
+    </p>
+  </dd>
+</dl><!-- go/build -->
+
 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
   <dd>
     <p><!-- CL 293649 -->