renaming the disable inactive extruder feature

Marlin/Configuration.h

 #define DISABLE_Y false
 #define DISABLE_Z false
 #define DISABLE_E false // For all extruders
-#define DISABLE_UNSELECTED_E true //disable only not selected extruders and keep selected extruder active
+#define DISABLE_INACTIVE_EXTRUDER true //disable only inactive extruders and keep active extruder enabled
 
 #define INVERT_X_DIR true    // for Mendel set to false, for Orca set to true
 #define INVERT_Y_DIR false    // for Mendel set to true, for Orca set to false

Marlin/planner.cpp

   // Enable extruder(s)
   if(block->steps_e != 0)
   {
-    if (DISABLE_UNSELECTED_E) //enable only selected extruder
+    if (DISABLE_INACTIVE_EXTRUDER) //enable only selected extruder
     {
       switch(extruder)
       {