upspin: modify Time comment to make it clear it's not dependable
upspin: modify Time comment to make it clear it's not dependable

Response to a post-commit code review.

Change-Id: I200ac8e423075f2b63c95276f3c5400f3f1aee53
Reviewed-on: https://upspin-review.git.corp.google.com/1496
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin