bufio: put notes about len(p) together

bufio: put notes about len(p) together

CL 145577 added the part about io.ReadFull to read len(p)
but it should be next to the existing sentence about not
reading len(p) bytes.

Change-Id: Idfa037c59a3085d44d5da6129188473db0e96d23
Reviewed-on: https://go-review.googlesource.com/c/148903
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>