px4_module: document that the subcategory is optional

src/platforms/px4_module.h

@@ -473,7 +473,7 @@ __EXPORT void PRINT_MODULE_DESCRIPTION(const char *description);
 __EXPORT void PRINT_MODULE_USAGE_NAME(const char *executable_name, const char *category);
 
 /**
- * @brief Specify a subcategory.
+ * @brief Specify a subcategory (optional).
  * @param subcategory e.g. if the category is 'driver', subcategory can be 'distance_sensor'
  */
 __EXPORT void PRINT_MODULE_USAGE_SUBCATEGORY(const char *subcategory);