Merge pull request #8650 from electron/will-attach-webview-docs

[ci skip] Improve docs for `will-attach-webview` event

docs/api/web-contents.md

@@ -516,6 +516,10 @@ Returns:
 Emitted when a `<webview>`'s web contents is being attached to this web
 contents. Calling `event.preventDefault()` will destroy the guest page.
 
+This event can be used to configure `webPreferences` for the `webContents`
+of a `<webview>` before it's loaded, and provides the ability to set settings
+that can't be set via `<webview>` attributes.
+
 ### Instance Methods
 
 #### `contents.loadURL(url[, options])`