docs: explain the null webContents case in permission checks (#30686)

* docs: explain the null webContents case in permission checks

* Update docs/api/session.md

Co-authored-by: Erick Zhao <erick@hotmail.ca>

Co-authored-by: Samuel Attard <samuel.r.attard@gmail.com>
Co-authored-by: Samuel Attard <sam@electronjs.org>
Co-authored-by: Erick Zhao <erick@hotmail.ca>

docs/api/session.md

@@ -531,7 +531,7 @@ session.fromPartition('some-partition').setPermissionRequestHandler((webContents
 #### `ses.setPermissionCheckHandler(handler)`
 
 * `handler` Function\<Boolean> | null
-  * `webContents` ([WebContents](web-contents.md) | null) - WebContents checking the permission.  Please note that if the request comes from a subframe you should use `requestingUrl` to check the request origin.  Cross origin sub frames making permission checks will pass a `null` webContents to this handler.  You should use `embeddingOrigin` and `requestingOrigin` to determine what origin the owning frame and the requesting frame are on respectively.
+  * `webContents` ([WebContents](web-contents.md) | null) - WebContents checking the permission.  Please note that if the request comes from a subframe you should use `requestingUrl` to check the request origin.  All cross origin sub frames making permission checks will pass a `null` webContents to this handler, while certain other permission checks such as `notifications` checks will always pass `null`.  You should use `embeddingOrigin` and `requestingOrigin` to determine what origin the owning frame and the requesting frame are on respectively.
   * `permission` String - Type of permission check.  Valid values are `midiSysex`, `notifications`, `geolocation`, `media`,`mediaKeySystem`,`midi`, `pointerLock`, `fullscreen`, `openExternal`, or `serial`.
   * `requestingOrigin` String - The origin URL of the permission check
   * `details` Object - Some properties are only available on certain permission types.