Add comment for the `matrix_client_element_logout_redirect_url` variable

roles/custom/matrix-client-element/defaults/main.yml

@@ -319,4 +319,6 @@ matrix_client_element_location_sharing_map_style_content_sources_localsource_til
 # Leave blank, if map does not require attribution.
 matrix_client_element_location_sharing_map_style_content_sources_localsource_attribution: ""
 
+# Optional URL to redirect the user to after they have logged out.
+# See https://github.com/element-hq/element-web/blob/develop/docs/config.md#sso-setup
 matrix_client_element_logout_redirect_url: ''