io: clarify Copy docs regarding error handling

io: clarify Copy docs regarding error handling

"returns ... the first error" was misleading or at least confusing:
in case a Read results in an error with non-zero bytes read, and the
subsequent Write also results in an error, the error from Write is
returned, which is the second one (in the temporal dimension).

Fixes #9744

Change-Id: If8925a701e4fae820cd9df7446503403fc0785d4
Reviewed-on: https://go-review.googlesource.com/3686
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>