Comment G26 properly

Marlin/src/gcode/bedlevel/G26.cpp

@@ -547,6 +547,24 @@ float valid_trig_angle(float d) {
  *
  * Used to interactively edit the mesh by placing the
  * nozzle in a problem area and doing a G29 P4 R command.
+ *
+ * Parameters:
+ *
+ *  B  Bed Temperature
+ *  C  Continue from the Closest mesh point
+ *  D  Disable leveling before starting
+ *  F  Filament diameter
+ *  H  Hotend Temperature
+ *  K  Keep heaters on when completed
+ *  L  Layer Height
+ *  O  Ooze extrusion length
+ *  P  Prime length
+ *  Q  Retraction multiplier
+ *  R  Repetitions (number of grid points)
+ *  S  Nozzle Size (diameter) in mm
+ *  U  Random deviation (50 if no value given)
+ *  X  X position
+ *  Y  Y position
  */
 void GcodeSuite::G26() {
   SERIAL_ECHOLNPGM("G26 command started. Waiting for heater(s).");