docs: add docs for configuration schema

docs/conf/writing.md

@@ -97,10 +97,10 @@ order:
 - If no config flags are provided, `app-config.local.yaml` has higher priority
   than `app-config.yaml`.
 
-## Secrets
+## Secrets and Dynamic Data
 
-Secrets are supported via special secret keys that are prefixed with `$`, which
-in turn provide a number of different ways to read in secrets. To load a
+Secrets are supported via special data loading keys that are prefixed with `$`,
+which in turn provide a number of different ways to read in secrets. To load a
 configuration value as a secret, supply an object with one of the special secret
 keys, for example `$env` or `$file`. A full list of supported secret keys can be
 found below. For example, the following will read the config key
@@ -117,10 +117,6 @@ will return the value of the environment variable `MY_SECRET_KEY` when the
 backend started up. All secrets are loaded at startup, so changing the contents
 of secret files or environment variables will not be reflected at runtime.
 
-Note that secrets will never be included in the frontend bundle or development
-builds. When loading configuration you have to explicitly enable reading of
-secrets, which is only done for the backend configuration.
-
 As hinted at, secrets can be loaded from a bunch of different sources, and can
 be extended with more. Below is a list of the currently supported methods for
 loading secrets.