doc: fix description of Duration.Round

Round doesn't always round away from zero.

Feel free to suggest better wording.

Change-Id: I52b3815013fff4c656ae8d861521c342bc4135b7
Reviewed-on: https://go-review.googlesource.com/46032Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.9.html

@@ -798,7 +798,7 @@ CL 40331: https://golang.org/cl/40331: cmd/link,runtime/cgo: enable PT_TLS gener
       <a href="/pkg/time/#Duration.Round"><code>Duration.Round</code></a>
       and 
       <a href="/pkg/time/#Duration.Truncate"><code>Duration.Truncate</code></a>
-      handle rounding durations away from and towards zero, respectively.
+      handle rounding and truncating durations to multiples of a given duration.
     </p>
 
     <p><!-- CL 35710 -->