api: update onDidChangeWindowState wording (microsoft#211184)

Author: connor4312

src/vscode-dts/vscode.d.ts

@@ -10456,7 +10456,7 @@ declare module 'vscode' {
 		export const state: WindowState;
 
 		/**
-		 * An {@link Event} which fires when the focus state of the current window
+		 * An {@link Event} which fires when the focus or activity state of the current window
 		 * changes. The value of the event represents whether the window is focused.
 		 */
 		export const onDidChangeWindowState: Event<WindowState>;