io: simplify Examples

- CopyN: 5 creates ambiguity with respect to whitespace and upperbound
- TeeReader less boilerplate and displays a common usage of it
- SectionReader_* all sections unified to 5:17 for clarity
- SectionReader_Seek uses io.Copy to stdout like other examples
- Seeker_Seek remove useless prints
- Pipe print reader like other examples

Updates #36417

Change-Id: Ibd01761d5a5786cdb1ea934f7a98f8302430c8a5
GitHub-Last-Rev: 4c17f9a8e32d89743b7eaec7c52032256972cc0a
GitHub-Pull-Request: golang/go#38379
Reviewed-on: https://go-review.googlesource.com/c/go/+/227868


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>