New and improved lock LED callbacks (#7215)

* New and improved lock LED callbacks

* Include stdbool

* Update documentation

* Use full function signatures and add keyboard-level example

quantum/template/base/keymaps/default/keymap.c

@@ -63,4 +63,6 @@ void matrix_init_user(void) {}
 
 void matrix_scan_user(void) {}
 
-void led_set_user(uint8_t usb_led) {}
+bool led_update_user(led_t led_state) {
+    return true;
+}