Restore M928, M999

Documentation/GCodes.md

@@ -126,6 +126,8 @@ M362 - SCARA calibration: Move to cal-position PsiA (0 deg calibration)
 M363 - SCARA calibration: Move to cal-position PsiB (90 deg calibration - steps per degree)
 M364 - SCARA calibration: Move to cal-position PSIC (90 deg to Theta calibration position)
 M365 - SCARA calibration: Scaling factor, X, Y, Z axis
+M928 - Start SD logging (M928 filename.g) - ended by M29
+M999 - Restart after being stopped by error
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