io/fs: add examples for Glob,ReadFile and ValidPath

Change-Id: I8451179bc0fa88b7e60afbc6fd9e06a22a94f3aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/673835
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

src/io/fs/example_test.go

@@ -9,8 +9,89 @@ import (
 	"io/fs"
 	"log"
 	"os"
+	"testing/fstest"
 )
 
+func ExampleGlob() {
+	fsys := fstest.MapFS{
+		"file.txt":        {},
+		"file.go":         {},
+		"dir/file.txt":    {},
+		"dir/file.go":     {},
+		"dir/subdir/x.go": {},
+	}
+
+	patterns := []string{
+		"*.txt",
+		"*.go",
+		"dir/*.go",
+		"dir/*/x.go",
+	}
+
+	for _, pattern := range patterns {
+		matches, err := fs.Glob(fsys, pattern)
+		if err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("%q matches: %v\n", pattern, matches)
+	}
+
+	// Output:
+	// "*.txt" matches: [file.txt]
+	// "*.go" matches: [file.go]
+	// "dir/*.go" matches: [dir/file.go]
+	// "dir/*/x.go" matches: [dir/subdir/x.go]
+}
+
+func ExampleReadFile() {
+	fsys := fstest.MapFS{
+		"hello.txt": {
+			Data: []byte("Hello, World!\n"),
+		},
+	}
+
+	data, err := fs.ReadFile(fsys, "hello.txt")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Print(string(data))
+
+	// Output:
+	// Hello, World!
+}
+
+func ExampleValidPath() {
+	paths := []string{
+		".",
+		"x",
+		"x/y/z",
+		"",
+		"..",
+		"/x",
+		"x/",
+		"x//y",
+		"x/./y",
+		"x/../y",
+	}
+
+	for _, path := range paths {
+		fmt.Printf("ValidPath(%q) = %t\n", path, fs.ValidPath(path))
+	}
+
+	// Output:
+	// ValidPath(".") = true
+	// ValidPath("x") = true
+	// ValidPath("x/y/z") = true
+	// ValidPath("") = false
+	// ValidPath("..") = false
+	// ValidPath("/x") = false
+	// ValidPath("x/") = false
+	// ValidPath("x//y") = false
+	// ValidPath("x/./y") = false
+	// ValidPath("x/../y") = false
+}
+
 func ExampleWalkDir() {
 	root := "/usr/local/go/bin"
 	fileSystem := os.DirFS(root)