strings: add a example for TrimFunc

Change-Id: I9c0c601ec5957475e949dcc4a8c2116724d01215
Reviewed-on: https://go-review.googlesource.com/48961Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/strings/example_test.go

@@ -234,6 +234,14 @@ func ExampleTrim() {
 	// Output: ["Achtung! Achtung"]
 }
 
+func ExampleTrimFunc() {
+	f := func(c rune) bool {
+		return !unicode.IsLetter(c) && !unicode.IsNumber(c)
+	}
+	fmt.Printf("[%q]", strings.TrimFunc("  Achtung1! Achtung2,...", f))
+	// Output: ["Achtung1! Achtung2"]
+}
+
 func ExampleMap() {
 	rot13 := func(r rune) rune {
 		switch {