docs(pivot): mark gridSize override internal

Author: damyanpetev

projects/igniteui-angular/src/lib/grids/pivot-grid/pivot-grid.component.ts

@@ -386,6 +386,7 @@ export class IgxPivotGridComponent extends IgxGridBaseDirective implements OnIni
         this._superCompactMode = value;
     }
 
+    /** @hidden @internal */
     public override get gridSize() {
         if (this.superCompactMode) {
             return Size.Small;