The previous wording implied that reads would return no error, rather
than EOF. It's convenient for users to know that Close() is equivalent
to CloseWithError(nil) because it can remove a branch from their error
handling code where they want to close the pipe in the appropriate way.
For example:

    https://github.com/jacobsa/gcloud/blob/6e9a8cec0a3f0834da3e9c9725dfe0bf79cccebb/gcs/bucket.go#L637-L643

Change-Id: I618bffe556eb518011e7ba5cdce1eb0ff536350e
Reviewed-on: https://go-review.googlesource.com/8152Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>