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>

src/bufio/bufio.go

@@ -186,9 +186,8 @@ func (b *Reader) Discard(n int) (discarded int, err error) {
 // It returns the number of bytes read into p.
 // The bytes are taken from at most one Read on the underlying Reader,
 // hence n may be less than len(p).
-// At EOF, the count will be zero and err will be io.EOF.
-//
 // To read exactly len(p) bytes, use io.ReadFull(b, p).
+// At EOF, the count will be zero and err will be io.EOF.
 func (b *Reader) Read(p []byte) (n int, err error) {
 	n = len(p)
 	if n == 0 {