Document missing server config options (#18122)

I was looking into the `USE_FROZEN_DICTS` option during the review of
#18103 (comment)
and noticed that there are several other server config options that
aren't in the docs.

changelog.d/18122.doc

@@ -0,0 +1 @@
+Document missing server config options (`daemonize`, `print_pidfile`, `user_agent_suffix`, `use_frozen_dicts`, `manhole`).

docs/usage/configuration/config_documentation.md

@@ -162,6 +162,53 @@ Example configuration:
 pid_file: DATADIR/homeserver.pid
 ```
 ---
+### `daemonize`
+
+Specifies whether Synapse should be started as a daemon process. If Synapse is being
+managed by [systemd](../../systemd-with-workers/), this option must be omitted or set to
+`false`.
+
+This can also be set by the `--daemonize` (`-D`) argument when starting Synapse.
+
+See `worker_daemonize` for more information on daemonizing workers.
+
+Example configuration:
+```yaml
+daemonize: true
+```
+---
+### `print_pidfile`
+
+Print the path to the pidfile just before daemonizing. Defaults to false.
+
+This can also be set by the `--print-pidfile` argument when starting Synapse.
+
+Example configuration:
+```yaml
+print_pidfile: true
+```
+---
+### `user_agent_suffix`
+
+A suffix that is appended to the Synapse user-agent (ex. `Synapse/v1.123.0`). Defaults
+to None
+
+Example configuration:
+```yaml
+user_agent_suffix: " (I'm a teapot; Linux x86_64)"
+```
+---
+### `use_frozen_dicts`
+
+Determines whether we should freeze the internal dict object in `FrozenEvent`. Freezing
+prevents bugs where we accidentally share e.g. signature dicts. However, freezing a
+dict is expensive. Defaults to false.
+
+Example configuration:
+```yaml
+use_frozen_dicts: true
+```
+---
 ### `web_client_location`
 
 The absolute URL to the web client which `/` will redirect to. Defaults to none.
@@ -595,6 +642,17 @@ listeners:
       - names: [client, federation]
 ```
 
+---
+### `manhole`
+
+Turn on the Twisted telnet manhole service on the given port. Defaults to none.
+
+This can also be set by the `--manhole` argument when starting Synapse.
+
+Example configuration:
+```yaml
+manhole: 1234
+```
 ---
 ### `manhole_settings`