flag: document Nflag function

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4279065

src/pkg/flag/flag.go

@@ -289,6 +289,7 @@ func failf(format string, a ...interface{}) {
 	os.Exit(2)
 }
 
+// NFlag returns the number of flags that have been set.
 func NFlag() int { return len(flags.actual) }
 
 // Arg returns the i'th command-line argument.  Arg(0) is the first remaining argument