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/56690Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/time.go

@@ -1383,7 +1383,7 @@ func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) T
 }
 
 // Truncate returns the result of rounding t down to a multiple of d (since the zero time).
-// If d <= 0, Truncate returns t unchanged.
+// If d <= 0, Truncate returns t stripped of any monotonic clock reading but otherwise unchanged.
 //
 // Truncate operates on the time as an absolute duration since the
 // zero time; it does not operate on the presentation form of the
@@ -1400,7 +1400,7 @@ func (t Time) Truncate(d Duration) Time {
 
 // Round returns the result of rounding t to the nearest multiple of d (since the zero time).
 // The rounding behavior for halfway values is to round up.
-// If d <= 0, Round returns t unchanged.
+// If d <= 0, Round returns t stripped of any monotonic clock reading but otherwise unchanged.
 //
 // Round operates on the time as an absolute duration since the
 // zero time; it does not operate on the presentation form of the