Commit dd5e14a7 ensured that no data
could be read for header-only files regardless of what the Header.Size
said. We should document this fact in Reader.Read.

Updates #13647

Change-Id: I4df9a2892bc66b49e0279693d08454bf696cfa31
Reviewed-on: https://go-review.googlesource.com/17913Reviewed-by: Russ Cox <rsc@golang.org>