docs: add comment

tool/setup_uid.c

@@ -38,6 +38,18 @@ bool check_user(char *filename, char *username, unsigned long *new_start) {
 }
 
 void insert_user(char *filename, char *username, unsigned long start) {
+    /*
+     * Insert the user into uid file
+     * It will setup the next start uid automatically
+     * == it will change /etc/subuid or /etc/subgid ==
+     *
+     * Args:
+     *      filename (char *): means the uid file (subuid or subgid).
+     *      username (char *): the user who run this excutable file.
+     *      new_start (unsigned long ): it's a value, means the next start uid
+     * Return:
+     *      No
+     */
     FILE *fp = fopen(filename, "a");
     
     if (fp == NULL) {