add comments to new added code

common/cmdarg/cmdarg.go

@@ -2,12 +2,14 @@ package cmdarg
 
 import "strings"
 
+// Arg is used by flag to accept multiple argument.
 type Arg []string
 
 func (c *Arg) String() string {
 	return strings.Join([]string(*c), " ")
 }
 
+// Set is the method flag package calls
 func (c *Arg) Set(value string) error {
 	*c = append(*c, value)
 	return nil