time: fix documentation of Round, Truncate behavior for d <= 0

time: fix documentation of Round, Truncate behavior for d <= 0

Saying that they return t unchanged is misleading, because they return
a modified t, stripped of any monotonic clock reading, as of Go 1.9.

Fixes #21485.

Change-Id: Icddf8813aed3d687fcefcd2fe542829438be6a0a
Reviewed-on: https://go-review.googlesource.com/56690
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Ian Lance Taylor <iant@golang.org>