Many times when using bufio.Reader I imagine, incorrectly, that it
implements the retry loop itself, being a high-level buffered wrapper
around, say, a file descriptor prone to short reads. This comment
would have saved me much time.

Change-Id: I34c790e0d7c1515430a76d02ce4739b586a36ba7
Reviewed-on: https://go-review.googlesource.com/c/145577Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>