Introduce the presence of the monotonic time reading first,
before the paragraph about comparison that mentions it multiple times.

Change-Id: I91e31e118be013eee6c258163a1bb2cb42501527
Reviewed-on: https://go-review.googlesource.com/76010
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>