path, path/filepath: add Join example with joined rooted path

This makes clear that Go's path.Join and filepath.Join are different
from the Python os.path.join (and perhaps others).

Requested in private mail.

Change-Id: Ie5dfad8a57f9baa5cca31246af1fd4dd5b1a64ee
Reviewed-on: https://go-review.googlesource.com/20711Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/path/example_test.go

@@ -54,7 +54,14 @@ func ExampleIsAbs() {
 
 func ExampleJoin() {
 	fmt.Println(path.Join("a", "b", "c"))
-	// Output: a/b/c
+	fmt.Println(path.Join("a", "b/c"))
+	fmt.Println(path.Join("a/b", "c"))
+	fmt.Println(path.Join("a/b", "/c"))
+	// Output:
+	// a/b/c
+	// a/b/c
+	// a/b/c
+	// a/b/c
 }
 
 func ExampleSplit() {

src/path/filepath/example_unix_test.go

@@ -65,3 +65,17 @@ func ExampleSplit() {
 	// 	dir: "/usr/local//"
 	// 	file: "go"
 }
+
+func ExampleJoin() {
+	fmt.Println("On Unix:")
+	fmt.Println(filepath.Join("a", "b", "c"))
+	fmt.Println(filepath.Join("a", "b/c"))
+	fmt.Println(filepath.Join("a/b", "c"))
+	fmt.Println(filepath.Join("a/b", "/c"))
+	// Output:
+	// On Unix:
+	// a/b/c
+	// a/b/c
+	// a/b/c
+	// a/b/c
+}