io: Add example to io.Seeker's Seek() method.

io: Add example to io.Seeker's Seek() method.

While there's an example for SectionReader.Seek, if someone is
seeking documentation specifically about Seeker.Seek, they may
not immediately find the SectionReader example. Offset and whence
may not be entirely intuitive to new developers either, so include
examples of both positive/negative offsets and SeekStart/SeekEnd.

Change-Id: I5b7442ccf683d9706e9261c11bc0ea31a1ac21d4
Reviewed-on: https://go-review.googlesource.com/48873
Reviewed-by: Kevin Burke <kev@inburke.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>