bufio: clarify Read docs

Or at least make them true.

Fixes #12237.

Change-Id: I3c92a07233b2174c5731d6fa7fbb9ca3a97beb6b
Reviewed-on: https://go-review.googlesource.com/17237Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bufio/bufio.go

@@ -179,7 +179,7 @@ func (b *Reader) Discard(n int) (discarded int, err error) {
 
 // Read reads data into p.
 // It returns the number of bytes read into p.
-// It calls Read at most once on the underlying Reader,
+// 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.
 func (b *Reader) Read(p []byte) (n int, err error) {