docs: allowpopups attribute

docs/api/web-view-tag.md

@@ -149,6 +149,14 @@ This value can only be modified before the first navigation, since the session
 of an active renderer process cannot change. Subsequent attempts to modify the
 value will fail with a DOM exception.
 
+### `allowpopups`
+
+```html
+<webview src="https://www.github.com/" allowpopups></webview>
+```
+
+If "on", the guest page will be allowed to open new windows.
+
 ## Methods
 
 The `webview` tag has the following methods:
@@ -496,7 +504,7 @@ Returns:
 * `url` String
 * `frameName` String
 * `disposition` String - Can be `default`, `foreground-tab`, `background-tab`,
-  `new-window` and `other`
+  `new-window` and `other`.
 
 Fired when the guest page attempts to open a new browser window.