docs(esf): add tree-grid LOD section …

IgniteUI/igniteui-angular#5448

en/components/grids_templates/excel_style_filtering.md

@@ -374,13 +374,28 @@ public columnValuesStrategy = (column: IgxColumnComponent,
 </div>
 }
 @@if (igxName === 'IgxTreeGrid') {
-<!-- <div class="sample-container loading" style="height:900px">
-    <iframe id="tree-grid-sample-3-iframe" data-src='{environment:demosBaseUrl}/tree-grid/treegrid-excel-style-filtering-sample-3' width="100%" height="100%" seamless frameborder="0" class="lazyload"></iframe>
+```html
+<igx-tree-grid #treeGrid [data]="data" [filterMode]="'excelStyleFilter'" [uniqueColumnValuesStrategy]="columnValuesStrategy">
+    ...
+</igx-tree-grid>
+```
+
+```typescript
+public columnValuesStrategy = (column: IgxColumnComponent,
+                               columnExprTree: IFilteringExpressionsTree,
+                               done: (uniqueValues: any[]) => void) => {
+    // Get specific column data.
+    this.remoteValuesService.getColumnData(column, columnExprTree, uniqueValues => done(uniqueValues));
+}
+```
+
+<div class="sample-container loading" style="height:800px">
+    <iframe id="tree-grid-esf-loadOnDemand-iframe" data-src='{environment:demosBaseUrl}/tree-grid/treegrid-excel-style-filtering-load-on-demand' width="100%" height="100%" seamless frameborder="0" class="lazyload"></iframe>
 </div>
 <br/>
 <div>
-<button data-localize="stackblitz" disabled class="stackblitz-btn" data-iframe-id="tree-grid-sample-3-iframe" data-demos-base-url="{environment:demosBaseUrl}">view on stackblitz</button>
-</div> -->
+<button data-localize="stackblitz" disabled class="stackblitz-btn" data-iframe-id="tree-grid-esf-loadOnDemand-iframe" data-demos-base-url="{environment:demosBaseUrl}">view on stackblitz</button>
+</div>
 }
 @@if (igxName === 'IgxHierarchicalGrid') {
 ```html
@@ -425,12 +440,12 @@ this.remoteValuesService.getColumnData(
 In order to provide a custom loading template for the excel style filtering, we can use the `igxExcelStyleLoading` directive:
 
 ```html
-<igx-grid #grid1 [data]="data" [filterMode]="'excelStyleFilter'" [uniqueColumnValuesStrategy]="columnValuesStrategy">
+<@@igSelector [data]="data" [filterMode]="'excelStyleFilter'" [uniqueColumnValuesStrategy]="columnValuesStrategy">
     ...
     <ng-template igxExcelStyleLoading>
         Loading ...
     </ng-template>
-</igx-grid>
+</@@igSelector>
 ```
 
 ### API References