MANUAL_STEPPER STOP_ON_ENDSTOP and SYNC description update

[dfrog3]

Added a line in the MANUAL_STEPPER gcode refrence that explains how the STOP_ON_ENDSTOP setting ignores the SYNC setting. I think this will be useful to users so they can better understand the limmitations of the MANUAL_STEPPER command. This behavior can be found in manual_stepper.cmd_MANUAL_STEPPER line 96.

[JamesH1978]

Thank you for submitting a PR, pleas refer to point 3 in "What to expect in a review" in https://github.com/Klipper3d/klipper/blob/master/docs/CONTRIBUTING.md and provide a signed off by line.

Thanks James

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