Description of agent_t::so_current_state() and agent_t::m_current_state_ptr has to be extended

If an agent uses nested states then `agent_t::m_current_state_ptr` will hold a pointer to inner-most state. This behaviour has to be described in Doxygen comments for `agent_t::m_current_state_ptr` and in the description for `agent_t::so_current_state()` method.