[update] descriptions of the pdf() and png() methods

Author: mafanya23

docs/api/export/pdf_method.md

@@ -17,9 +17,13 @@ To avoid problems during export, all images for Diagram shapes must be set eithe
 ### Usage
 
 ~~~jsx
-pdf(config?: object): void;
+pdf(config?: object): Promise<void>;
 ~~~
 
+### Returns
+
+A promise of data export
+
 ### Parameters
 
 - `config` - (optional) an object with export settings. You can specify the following settings for export to PDF:
@@ -53,20 +57,26 @@ pdf(config?: object): void;
 
 ### Example
 
-~~~jsx {7,10-13}
+~~~jsx 
 const diagram = new dhx.Diagram("diagram_container", {
     // config options
 });
 diagram.data.parse(data);
 
 // default export
-diagram.export.pdf();
+diagram.export.pdf()
+    .then(() => console.log("success"))
+    .catch(() => console.log("failure"))
+    .finally(() => console.log("finished"));
 
 // export with config settings
 diagram.export.pdf({
     url: "https://export.dhtmlx.com/diagram/pdf/5.0.0",
     name:"result_pdf"
-});
+})
+    .then(() => console.log("success"))
+    .catch(() => console.log("failure"))
+    .finally(() => console.log("finished"));
 ~~~
 
 ### Details

docs/api/export/png_method.md

@@ -17,9 +17,13 @@ To avoid problems during export, all images for Diagram shapes must be set eithe
 ### Usage
 
 ~~~jsx
-png(config?: object): void;
+png(config?: object): Promise<void>;
 ~~~
 
+### Returns
+
+A promise of data export
+
 ### Parameters
 
 - `config` - (optional) an object with export settings. You can specify the following settings for export to PNG:
@@ -30,19 +34,25 @@ png(config?: object): void;
 
 ### Example
 
-~~~jsx {7,10-13}
+~~~jsx 
 const diagram = new dhx.Diagram("diagram_container", {
     // config options
 });
 diagram.data.parse(data);
 
 // default export
-diagram.export.png();
+diagram.export.png()
+    .then(() => console.log("success"))
+    .catch(() => console.log("failure"))
+    .finally(() => console.log("finished"));
 
 // export with config settings
 diagram.export.png({
     name: "result_png"
-});
+})
+    .then(() => console.log("success"))
+    .catch(() => console.log("failure"))
+    .finally(() => console.log("finished"));
 ~~~
 
 **Related articles**:  [Exporting Diagram](../../../guides/data_export/)