fmt: document that Scanf returns an error the same as Scan

No semantic change.

Fixes #8708.

Change-Id: Ieda04a86a19bb69bfc2519d381a2f025e7cb8279
Reviewed-on: https://go-review.googlesource.com/9740Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/scan.go

@@ -81,6 +81,7 @@ func Scanln(a ...interface{}) (n int, err error) {
 // Scanf scans text read from standard input, storing successive
 // space-separated values into successive arguments as determined by
 // the format.  It returns the number of items successfully scanned.
+// If that is less than the number of arguments, err will report why.
 func Scanf(format string, a ...interface{}) (n int, err error) {
 	return Fscanf(os.Stdin, format, a...)
 }