Clarify a sanity check

Marlin/src/inc/SanityCheck.h

  */
 #if ENABLED(MIXING_EXTRUDER)
   #if EXTRUDERS > 1
-    #error "MIXING_EXTRUDER currently only supports one extruder."
+    #error "For MIXING_EXTRUDER set MIXING_STEPPERS > 1 instead of EXTRUDERS > 1."
   #elif MIXING_STEPPERS < 2
     #error "You must set MIXING_STEPPERS >= 2 for a mixing extruder."
   #elif ENABLED(FILAMENT_SENSOR)