improve SmartPort::get_mutex documentation

Author: liam-teale

include/pros/devices/port.hpp

@@ -73,15 +73,15 @@ class SmartPort {
     /**
      * @brief Get the mutex for the given port
      *
-     * @todo decide whether this should go in a source file
+     * @note if the port is not valid (index > 31), a reference to a reserved mutex is returned.
      *
      * @return pros::RecursiveMutex&
      */
     pros::RecursiveMutex& get_mutex() const {
         if (this->is_valid()) {
             return m_mutexes.at(m_index);
         } else {
-            return *m_mutexes.end();
+            return m_mutexes.at(32);
         }
     }