doc/go1.12: note that Go 1.12 is the last release to include godoc

Updates #30029

Change-Id: I88e09035d675e7a6855ada0262eb42636c9822cc
Reviewed-on: https://go-review.googlesource.com/c/162417
Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.12.html

@@ -303,7 +303,9 @@ <h3 id="godoc"><code>godoc</code> and <code>go</code> <code>doc</code></h3>
 <p>
   In Go 1.12, <code>godoc</code> no longer has a command-line interface and
   is only a web server. Users should use <code>go</code> <code>doc</code>
-  for command-line help output instead.
+  for command-line help output instead. Go 1.12 is the last release that will
+  include the <code>godoc</code> webserver; in Go 1.13 it will be available
+  via <code>go</code> <code>get</code>.
 </p>
 
 <p><!-- CL 141977 -->