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/8152Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/io/pipe.go

@@ -168,7 +168,10 @@ func (w *PipeWriter) Close() error {
 }
 
 // CloseWithError closes the writer; subsequent reads from the
-// read half of the pipe will return no bytes and the error err.
+// read half of the pipe will return no bytes and the error err,
+// or EOF if err is nil.
+//
+// CloseWithError always returns nil.
 func (w *PipeWriter) CloseWithError(err error) error {
 	w.p.wclose(err)
 	return nil