Cheatsheet: Add docs about reset token expiration

docs/configuration/cheatsheet.md

@@ -63,6 +63,7 @@ To add configuration to your config file, you can copy it from the base config.
 * `external_user_synchronization`: Enabling following/followers counters synchronization for external users.
 * `cleanup_attachments`: Remove attachments along with statuses. Does not affect duplicate files and attachments without status. Enabling this will increase load to database when deleting statuses on larger instances.
 * `show_reactions`: Let favourites and emoji reactions be viewed through the API (default: `true`).
+* `password_reset_token_validity`: The time after which reset tokens aren't accepted anymore, in seconds (default: one day).
 
 ## Welcome
 * `direct_message`: - welcome message sent as a direct message.