add comments

libraries/entities/src/EntitySimulation.cpp

@@ -43,6 +43,7 @@ void EntitySimulation::updateEntities() {
 }
 
 void EntitySimulation::removeEntityFromInternalLists(EntityItemPointer entity) {
+    // protected: _mutex lock is guaranteed
     // remove from all internal lists except _deadEntitiesToRemoveFromTree
     _entitiesToSort.remove(entity);
     _simpleKinematicEntities.remove(entity);
@@ -144,6 +145,7 @@ void EntitySimulation::sortEntitiesThatMoved() {
 }
 
 void EntitySimulation::addEntityToInternalLists(EntityItemPointer entity) {
+    // protected: _mutex lock is guaranteed
     if (entity->isMortal()) {
         _mortalEntities.insert(entity);
         uint64_t expiry = entity->getExpiry();

libraries/physics/src/PhysicalEntitySimulation.cpp

@@ -171,6 +171,8 @@ void PhysicalEntitySimulation::processChangedEntity(const EntityItemPointer& ent
 }
 
 void PhysicalEntitySimulation::processDeadEntities() {
+    // Note: this override is a complete rewite of the base class's method because we cannot assume all entities
+    // are domain entities, and the consequence of trying to delete a domain-entity in this case is very different.
     if (_deadEntitiesToRemoveFromTree.empty()) {
         return;
     }