Document help_url and help_encryption_url (#25549)

docs/config.md

@@ -144,6 +144,8 @@ complete re-branding/private labeling, a more personalised experience can be ach
     1. `title`: Required. Title to show at the top of the notice.
     2. `description`: Required. The description to use for the notice.
     3. `show_once`: Optional. If true then the notice will only be shown once per device.
+18. `help_url`: The URL to point users to for help with the app, defaults to `https://element.io/help`.
+19. `help_encrption_url`: The URL to point users to for help with encryption, defaults to `https://element.io/help#encryption`.
 
 ### `desktop_builds` and `mobile_builds`