Improve M906 description

Marlin/src/gcode/feature/trinamic/M906.cpp

@@ -29,8 +29,18 @@
 #include "../../../module/stepper_indirection.h"
 
 /**
- * M906: Set motor current in milliamps using axis codes X, Y, Z, E
+ * M906: Set motor current in milliamps.
- * Report driver currents when no axis specified
+ *
+ * Parameters:
+ *   X[current]  - Set mA current for X driver(s)
+ *   Y[current]  - Set mA current for Y driver(s)
+ *   Z[current]  - Set mA current for Z driver(s)
+ *   E[current]  - Set mA current for E driver(s)
+ *
+ *   I[index]    - Axis sub-index (Omit or 0 for X, Y, Z; 1 for X2, Y2, Z2; 2 for Z3.)
+ *   T[index]    - Extruder index (Zero-based. Omit for E0 only.)
+ *
+ * With no parameters report driver currents.
  */
 void GcodeSuite::M906() {
   #define TMC_SAY_CURRENT(Q) tmc_get_current(stepper##Q)