strings: Add FieldsFunc example.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/42360043

src/pkg/strings/example_test.go

@@ -7,6 +7,7 @@ package strings_test
 import (
 	"fmt"
 	"strings"
+	"unicode"
 )
 
 func ExampleFields() {
@@ -14,6 +15,14 @@ func ExampleFields() {
 	// Output: Fields are: ["foo" "bar" "baz"]
 }
 
+func ExampleFieldsFunc() {
+	f := func(c rune) bool {
+		return !unicode.IsLetter(c) && !unicode.IsNumber(c)
+	}
+	fmt.Printf("Fields are: %q", strings.FieldsFunc("  foo1;bar2,baz3...", f))
+	// Output: Fields are: ["foo1" "bar2" "baz3"]
+}
+
 func ExampleContains() {
 	fmt.Println(strings.Contains("seafood", "foo"))
 	fmt.Println(strings.Contains("seafood", "bar"))