doc: add code comments

os/gfile/gfile_z_example_cache_test.go

@@ -24,12 +24,14 @@ func ExampleGetContentsWithCache() {
 	// write contents
 	gfile.PutContents(tempFile, "goframe example content")
 
-	// read contents
+	// It reads the file content with cache duration of one minute,
+	// which means it reads from cache after then without any IO operations within on minute.
 	fmt.Println(gfile.GetContentsWithCache(tempFile, time.Minute))
 
 	// write new contents will clear its cache
 	gfile.PutContents(tempFile, "new goframe example content")
 
+	// There's some delay for cache clearing after file content change.
 	time.Sleep(time.Second * 1)
 
 	// read contents

os/gfile/gfile_z_example_contents_test.go

@@ -23,7 +23,8 @@ func ExampleGetContents() {
 	// write contents
 	gfile.PutContents(tempFile, "goframe example content")
 
-	// read contents
+	// It reads and returns the file content as string.
+	// It returns empty string if it fails reading, for example, with permission or IO error.
 	fmt.Println(gfile.GetContents(tempFile))
 
 	// Output:
@@ -41,7 +42,8 @@ func ExampleGetBytes() {
 	// write contents
 	gfile.PutContents(tempFile, "goframe example content")
 
-	// read contents
+	// It reads and returns the file content as []byte.
+	// It returns nil if it fails reading, for example, with permission or IO error.
 	fmt.Println(gfile.GetBytes(tempFile))
 
 	// Output:
@@ -56,7 +58,8 @@ func ExamplePutContents() {
 		tempFile = gfile.Join(tempDir, fileName)
 	)
 
-	// write contents
+	// It creates and puts content string into specifies file path.
+	// It automatically creates directory recursively if it does not exist.
 	gfile.PutContents(tempFile, "goframe example content")
 
 	// read contents

os/gfile/gfile_z_example_replace_test.go

@@ -50,7 +50,7 @@ func ExampleReplaceFileFunc() {
 	// read contents
 	fmt.Println(gfile.GetContents(tempFile))
 
-	// replace by yourself
+	// It replaces content directly by file path and callback function.
 	gfile.ReplaceFileFunc(func(path, content string) string {
 		// Replace with regular match
 		reg, _ := regexp.Compile(`\d{3}`)
@@ -78,6 +78,7 @@ func ExampleReplaceDir() {
 	// read contents
 	fmt.Println(gfile.GetContents(tempFile))
 
+	// It replaces content of all files under specified directory recursively.
 	gfile.ReplaceDir("content", "replace word", tempDir, "gflie_example.txt", true)
 
 	// read contents
@@ -102,7 +103,7 @@ func ExampleReplaceDirFunc() {
 	// read contents
 	fmt.Println(gfile.GetContents(tempFile))
 
-	// replace by yourself
+	// It replaces content of all files under specified directory with custom callback function recursively.
 	gfile.ReplaceDirFunc(func(path, content string) string {
 		// Replace with regular match
 		reg, _ := regexp.Compile(`\d{3}`)

os/gfile/gfile_z_example_scan_test.go

@@ -55,7 +55,7 @@ func ExampleScanDirFile() {
 	gfile.PutContents(tempSubFile, "goframe example content")
 
 	// scans directory recursively exclusive of directories
-	list, _ := gfile.ScanDirFile(tempDir, "*", true)
+	list, _ := gfile.ScanDirFile(tempDir, "*.txt", true)
 	for _, v := range list {
 		fmt.Println(gfile.Basename(v))
 	}
@@ -116,7 +116,7 @@ func ExampleScanDirFileFunc() {
 	gfile.PutContents(tempSubFile, "goframe example content")
 
 	// scans directory recursively exclusive of directories
-	list, _ := gfile.ScanDirFileFunc(tempDir, "*", true, func(path string) string {
+	list, _ := gfile.ScanDirFileFunc(tempDir, "*.txt", true, func(path string) string {
 		// ignores some files
 		if gfile.Basename(path) == "gflie_example_ignores.txt" {
 			return ""