doc(tree-grid): type key inputs (#14487)

Author: damyanpetev

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

@@ -169,7 +169,7 @@ export class IgxTreeGridComponent extends IgxGridBaseDirective implements GridTy
      * @memberof IgxTreeGridComponent
      */
     @Input()
-    public childDataKey;
+    public childDataKey: string;
 
     /**
      * Sets the foreign key of the `IgxTreeGridComponent`.
@@ -181,7 +181,7 @@ export class IgxTreeGridComponent extends IgxGridBaseDirective implements GridTy
      * @memberof IgxTreeGridComponent
      */
     @Input()
-    public foreignKey;
+    public foreignKey: string;
 
     /**
      * Sets the key indicating whether a row has children.
@@ -196,7 +196,7 @@ export class IgxTreeGridComponent extends IgxGridBaseDirective implements GridTy
      * @memberof IgxTreeGridComponent
      */
     @Input()
-    public hasChildrenKey;
+    public hasChildrenKey: string;
 
     /**
      * Sets whether child records should be deleted when their parent gets deleted.