bytes: add examples for TrimLeft and TrimRight

Change-Id: Ib6d94f185dd43568cf97ef267dd51a09f43a402f
Reviewed-on: https://go-review.googlesource.com/51391Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/bytes/example_test.go

@@ -281,11 +281,23 @@ func ExampleMap() {
 	// Output: 'Gjnf oevyyvt naq gur fyvgul tbcure...
 }
 
+func ExampleTrimLeft() {
+	fmt.Print(string(bytes.TrimLeft([]byte("+ 005400"), "+0 ")))
+	// Output:
+	// 5400
+}
+
 func ExampleTrimSpace() {
 	fmt.Printf("%s", bytes.TrimSpace([]byte(" \t\n a lone gopher \n\t\r\n")))
 	// Output: a lone gopher
 }
 
+func ExampleTrimRight() {
+	fmt.Print(string(bytes.TrimRight([]byte("453gopher8257"), "0123456789")))
+	// Output:
+	// 453gopher
+}
+
 func ExampleToUpper() {
 	fmt.Printf("%s", bytes.ToUpper([]byte("Gopher")))
 	// Output: GOPHER