[add] header/footer config settings for pdf()/png() methods

Author: mafanya23

docs/api/export/pdf_method.md

@@ -48,6 +48,8 @@ pdf(config?: object): void;
             - `title` - (optional) the document name
             - `pageNumber` - (optional) the current page number
             - `totalPages` - (optional) the total number of pages in the document
+    - `header?: string` - (optional) an HTML template for the header in the exported file
+    - `footer?: string` - (optional) an HTML template for the footer in the exported file
 
 ### Example
 

docs/api/export/png_method.md

@@ -23,8 +23,10 @@ png(config?: object): void;
 ### Parameters
 
 - `config` - (optional) an object with export settings. You can specify the following settings for export to PNG:
-  - `url?: string` - the url of the service that executes export and returns an exported file. This setting is optional, you should use it only if you need to specify the path to your local export service. The default value is `https://export.dhtmlx.com/diagram/png/5.0.0`
-  - `name?: string` - the name of the exported file
+  - `url?: string` - (optional) the url of the service that executes export and returns an exported file. This setting is optional, you should use it only if you need to specify the path to your local export service. The default value is `https://export.dhtmlx.com/diagram/png/5.0.0`
+  - `name?: string` - (optional) the name of the exported file
+  - `header?: string` - (optional) an HTML template for the header in the exported file
+  - `footer?: string` - (optional) an HTML template for the footer in the exported file
 
 ### Example