time: improve Truncate and Round documentation

time: improve Truncate and Round documentation

Truncate and Round operate on absolute time, which means that
Truncate(Hour) may return a time with non-zero Minute(). Document that
more clearly, and remove the misleading example which suggests it is
safe.

Updates #16647

Change-Id: I930584ca030dd12849195d45e49ed2fb74e0c9ac
Reviewed-on: https://go-review.googlesource.com/28730
Reviewed-by: Ian Lance Taylor <iant@golang.org>