doc/go1.12: add notes about 'go doc -src'

Change-Id: Iaf67fcbb145277327e24150b29ff38f6c65f6a03
Reviewed-on: https://go-review.googlesource.com/c/155781Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.12.html

@@ -299,6 +299,11 @@ for {
   as the <code>godoc</code> command line used to do.
 </p>
 
+<p><!-- CL 140959 -->
+  <code>go</code> <code>doc</code> also now includes the <code>-src</code> flag,
+  which will show the target's original source code.
+</p>
+
 <h3 id="trace">Trace</h3>
 
 <p><!-- CL 60790 -->