docs: updated to reflect new --config flags

docs/conf/writing.md

@@ -55,20 +55,28 @@ picked up by the serve tasks of `@backstage/cli` for local development, and are
 injected by the entrypoint of the nginx container serving the frontend in a
 production build.
 
-## File Resolution
+## Configuration Files
 
 It is possible to have multiple configuration files, both to support different
 environments, but also to define configuration that is local to specific
-packages.
+packages. The configuration files to load are selected using a `--config <path>`
+flag, and it is possible to load any number of files. Paths are relative to the
+working directory of the executed process, for example `package/backend`. This
+means that to select a config file in the repo root when running the backend,
+you would use `--config ../../my-config.yaml`.
 
-All `app-config.yaml` files inside the monorepo root and package root are
-considered, as are files with additional `local` and environment affixes such as
-`development`, for example `app-config.local.yaml`,
-`app-config.production.yaml`, and `app-config.development.local.yaml`. Which
-environment config files are loaded is determined by the `APP_ENV` environment
-variable, or `NODE_ENV` if it is not set. Local configuration files are always
-loaded, but are meant for local development overrides and should typically be
-`.gitignore`'d.
+If no `config` flags are specified, the default behavior is to load
+`app-config.yaml` and, if it exists, `app-config.local.yaml` from the repo root.
+In the provided project setup, `app-config.local.yaml` is `.gitignore`'d, making
+it a good place to add config overrides and secrets for local development.
+
+Note that if any config flags are provided, the default `app-config.yaml` files
+are NOT loaded. To include them you need to explicitly include them with a flag,
+for example:
+
+```
+yarn start --config ../../app-config.yaml --config ../../app-config.staging.yaml
+```
 
 All loaded configuration files are merged together using the following rules:
 
@@ -84,10 +92,10 @@ order:
 
 - Configuration from the `APP_CONFIG_` environment variables has the highest
   priority, followed by files.
-- Files inside package directories have higher priority than those in the root
-  directory.
-- Files with environment affixes have higher priority than ones without.
-- Files with the `local` affix have higher priority than ones without.
+- Files loaded with config flags are ordered by priority, where the last flag
+  has the highest priority.
+- If no config flags are provided, `app-config.local.yaml` has higher priority
+  than `app-config.yaml`.
 
 ## Secrets