module documentation: add support for subcategories

src/platforms/common/module.cpp

@@ -64,6 +64,11 @@ void PRINT_MODULE_USAGE_NAME(const char *executable_name, const char *category)
 	PX4_INFO_RAW(" Commands:\n");
 }
 
+void PRINT_MODULE_USAGE_SUBCATEGORY(const char *subcategory)
+{
+	(void)subcategory;
+}
+
 void PRINT_MODULE_USAGE_NAME_SIMPLE(const char *executable_name, const char *category)
 {
 	PX4_INFO_RAW("Usage: %s [arguments...]\n", executable_name);

src/platforms/px4_module.h

@@ -472,6 +472,12 @@ __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.
+ * @param subcategory e.g. if the category is 'driver', subcategory can be 'distance_sensor'
+ */
+__EXPORT void PRINT_MODULE_USAGE_SUBCATEGORY(const char *subcategory);
+
 /**
  * @brief Prints the name for a command without any sub-commands (@see PRINT_MODULE_USAGE_NAME()).
  */