Add a warning on LA + S-Curve

Marlin/Configuration_adv.h

   //#define EXTRA_LIN_ADVANCE_K // Enable for second linear advance constants
   #define LIN_ADVANCE_K 0.22    // Unit: mm compression per 1mm/s extruder speed
   //#define LA_DEBUG            // If enabled, this will generate debug information output over USB.
+  //#define EXPERIMENTAL_SCURVE // Enable this option to permit S-Curve Acceleration
 #endif
 
 // @section leveling

Marlin/src/inc/SanityCheck.h

     WITHIN(LIN_ADVANCE_K, 0, 10),
     "LIN_ADVANCE_K must be a value from 0 to 10 (Changed in LIN_ADVANCE v1.5, Marlin 1.1.9)."
   );
+  #if ENABLED(S_CURVE_ACCELERATION) && DISABLED(EXPERIMENTAL_SCURVE)
+    #error "LIN_ADVANCE and S_CURVE_ACCELERATION may not play well together! Enable EXPERIMENTAL_SCURVE to continue."
+  #endif
 #endif
 
 /**