io: clarify the behavior of PipeWriter.CloseWithError(nil).
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/8152
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>