io: add a comment about how to turn a Reader into ByteReader
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,CAFxX/go,akutz/go,golang/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,CAFxX/go,golang/go,golang/go,akutz/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go