🧑‍💻 Clarify acceleration factor

Marlin/src/module/planner.cpp

   block->acceleration_steps_per_s2 = accel;
   block->acceleration = accel / steps_per_mm;
   #if DISABLED(S_CURVE_ACCELERATION)
-    block->acceleration_rate = (uint32_t)(accel * (sq(4096.0f) / (STEPPER_TIMER_RATE)));
+    block->acceleration_rate = (uint32_t)(accel * (float(1 << 24) / (STEPPER_TIMER_RATE)));
   #endif
   #if ENABLED(LIN_ADVANCE)
     if (block->use_advance_lead) {