io: add a comment about how to turn a Reader into ByteReader

Offered as an alternative to CL 221380, which was more
tutorial than necessary.

Update #37344

Change-Id: Ide673b0b97983c2c2319a9311dc3d0a10567e6c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/223097


Reviewed-by: Ian Lance Taylor <iant@golang.org>