Clarify mjs_addDefault usage and document correct default class naming

doc/includes/references.h

@@ -3463,6 +3463,10 @@ mjsTuple* mjs_addTuple(mjSpec* s);
 mjsKey* mjs_addKey(mjSpec* s);
 mjsPlugin* mjs_addPlugin(mjSpec* s);
 mjsDefault* mjs_addDefault(mjSpec* s, const char* classname, const mjsDefault* parent);
+// Example usage:
+// mjs_addDefault(spec, "myclass", parent);  // Named default
+// mjs_addDefault(spec, nullptr, parent);     // Only for the root unnamed default
+mjsDefault* mjs_addDefault(mjSpec* s, const char* classname, const mjsDefault* parent);
 const char* mjs_setToMotor(mjsActuator* actuator);
 const char* mjs_setToPosition(mjsActuator* actuator, double kp, double kv[1],
                               double dampratio[1], double timeconst[1], double inheritrange);

include/mujoco/mujoco.h

@@ -1566,6 +1566,10 @@ MJAPI mjsPlugin* mjs_addPlugin(mjSpec* s);
 
 // Add default.
 // Nullable: parent
+// Add a default class to the model. Pass a non-null classname for named defaults; only one unnamed default is allowed at the top level.
+// Example usage:
+// mjs_addDefault(spec, "myclass", parent);  // Named default
+// mjs_addDefault(spec, nullptr, parent);     // Only for the root unnamed default
 MJAPI mjsDefault* mjs_addDefault(mjSpec* s, const char* classname, const mjsDefault* parent);
 
 

src/user/user_api.h

@@ -157,7 +157,10 @@ MJAPI mjsKey* mjs_addKey(mjSpec* s);
 // Add plugin.
 MJAPI mjsPlugin* mjs_addPlugin(mjSpec* s);
 
-// Add default.
+// Add a default class to the model. Pass a non-null classname for named defaults; only one unnamed default is allowed at the top level.
+// Example usage:
+// mjs_addDefault(spec, "myclass", parent);  // Named default
+// mjs_addDefault(spec, nullptr, parent);     // Only for the root unnamed default
 MJAPI mjsDefault* mjs_addDefault(mjSpec* s, const char* classname, const mjsDefault* parent);