Make pollcat_loop_exit() a oneshot

include/pollcat/struct.h

@@ -95,7 +95,13 @@ struct pollcat *pollcat_iter_next (void);
 int pollcat_loop (int *poll_ret);
 
 
-/** Cause pollcat_loop() to return 0 on its next call
+/** Cause pollcat_loop() to return 0 on its next call only
+ *
+ *  \note  Each call of pollcat_loop_exit() causes pollcat_loop() to return 0 once;
+ *         pollcat_loop() then resets the flag to return nonzero until the next time
+ *         pollcat_loop_exit() has been called.  This allows the entire main loop to be
+ *         wrapped in an outer loop so tasks like updating firmware can be done without
+ *         affecting the pollcat loop.
  */
 void pollcat_loop_exit (void);