Document that the byte value returned by ReadByte() is meaningless
if its error != nil. Because io.Reader and io.ByteReader are similar in
name, this CL aims to clear up any ambiguity surrounding the returned
values, particularly where io.Reader is allowed to return both a
non-zero number of valid bytes and err == EOF.

Fixes #20825

Change-Id: I3a23c18c80c471c0caae3b4d2f6f8e547da0bed9
Reviewed-on: https://go-review.googlesource.com/46950Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>