doc/go1.13: mention '-o <directory>' support for 'go build'

Fixes golang#33720
Updates golang#14295

Change-Id: I9cb6e02bcaccd7971057315163d8810157d465bd
Reviewed-on: https://go-review.googlesource.com/c/go/+/190907
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

doc/go1.13.html

@@ -389,6 +389,13 @@ <h3 id="go-command">Go command</h3>
   from the compiled executable, to improve build reproducibility.
 </p>
 
+<p id="o-dir"><!-- CL 167679 -->
+  If the <code>-o</code> flag passed to <code>go</code> <code>build</code>
+  refers to an existing directory, <code>go</code> <code>build</code> will now
+  write executable files within that directory for <code>main</code> packages
+  matching its package arguments.
+</p>
+
 <p id="comma-separated-tags"><!-- CL 173438 -->
   The <code>go</code> <code>build</code> flag <code>-tags</code> now takes a
   comma-separated list of build tags, to allow for multiple tags in