doc: Add titles for browser-window and frameless-window.

docs/browser-window.md

@@ -1,4 +1,4 @@
-## Synopsis
+# browser-window
 
 The `BrowserWindow` class gives you ability to create a browser window, an
 example is:

docs/frameless-window.md

@@ -1,3 +1,5 @@
+# Frameless window
+
 A frameless window is a window that has no chrome.
 
 ## Create a frameless window