New jog cancel real-time command. Parser typo fix from last push.

- Added a new jog cancel real-time command. Rather than depending on a
feed hold to cancel a jogging motion, this realtime command can be used
instead. The main advantage is if a feed hold is used, you can
accidentally hold the machine right when Grbl returns to IDLE after
completing a jog. And the GUI doesn’t have to worry about tracking this
either.

- Fixed a typo in the g-code parser edits from the last push. Was
causing the G10 set coordinate system command to not work correctly.

- Updated the documentation with the jog cancel command.

grbl/protocol.c

@@ -276,9 +276,9 @@ void protocol_exec_rt_system()
         // to halt and cancel the remainder of the motion.
         if (rt_exec & EXEC_MOTION_CANCEL) {
           // MOTION_CANCEL only occurs during a CYCLE, but a HOLD and SAFETY_DOOR may been initiated beforehand
-          // to hold the CYCLE. If so, only flag that motion cancel is complete.
-          // NOTE: State is still STATE_CYCLE.
-          sys.suspend |= SUSPEND_MOTION_CANCEL;  // Indicate motion cancel when resuming.
+          // to hold the CYCLE. Motion cancel is valid for a single planner block motion only, while jog cancel
+          // will handle and clear multiple planner block motions.
+          if (!(sys.state & STATE_JOG)) { sys.suspend |= SUSPEND_MOTION_CANCEL; } // NOTE: State is STATE_CYCLE.
         }
 
         // Execute a feed hold with deceleration, if required. Then, suspend system.