path/filepath: add examples for SplitList and Rel.

R=golang-dev, bradfitz, minux.ma
CC=golang-dev
https://golang.org/cl/7291043

src/pkg/path/filepath/example_unix_test.go

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !windows,!plan9
+
+package filepath_test
+
+import (
+	"fmt"
+	"path/filepath"
+)
+
+func ExampleSplitList() {
+	fmt.Println("On Unix:", filepath.SplitList("/a/b/c:/usr/bin"))
+	// Output:
+	// On Unix: [/a/b/c /usr/bin]
+}
+
+func ExampleRel() {
+	paths := []string{
+		"/a/b/c",
+		"/b/c",
+		"./b/c",
+	}
+	base := "/a"
+
+	fmt.Println("On Unix:")
+	for _, p := range paths {
+		rel, err := filepath.Rel(base, p)
+		fmt.Printf("%q: %q %v\n", p, rel, err)
+	}
+
+	// Output:
+	// On Unix:
+	// "/a/b/c": "b/c" <nil>
+	// "/b/c": "../b/c" <nil>
+	// "./b/c": "" Rel: can't make b/c relative to /a
+}