net: consistently document deadline handling

After CL 228645 some mentions of the Deadline methods referred
to the Timeout method, and some to os.ErrDeadlineExceeded.
Stop referring to the Timeout method, to encourage ErrDeadlineExceeded.

For #31449

Change-Id: I27b8ff34f31798f38b06437546886af8cce98ca4
Reviewed-on: https://go-review.googlesource.com/c/go/+/239705


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>