* Update attribute names in code examples (* -> settings.*).
* Use `nix-shell -p` rather than `nix run` because the example won't
work with the current default Nix.
* Update config values for `element-web`.
* Fix link to `element-web` security considerations.
* Make the synapse expression even smaller and use callout-lists to
explain the code.
* Document how to correctly deploy the shared registration secret.
[1] https://spec.matrix.org/latest/client-server-api/#getwell-knownmatrixclient