doc/go1.14: mention new field go/build.Context.Dir

Updates #34860
Updates #36168
Updates #36878

Change-Id: I484d7fea5d77d6dcd451d4fdffe0c450eed73636
Reviewed-on: https://go-review.googlesource.com/c/go/+/217124
Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go1.14.html

@@ -482,6 +482,18 @@ <h2 id="library">Core library</h2>
 
 </dl><!-- encoding/json -->
 
+<dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
+  <dd>
+    <p><!-- CL 203820, 211657 -->
+      The <a href="/pkg/go/build/#Context"><code>Context</code></a>
+      type has a new field <code>Dir</code> which may be used to set
+      the working directory for the build.
+      The default is the current directory of the running process.
+      In module mode, this is used to locate the main module.
+    </p>
+  </dd>
+</dl><!-- go/build -->
+
 <dl id="log"><dt><a href="/pkg/log/">log</a></dt>
   <dd>
     <p><!-- CL 186182 -->