Add documentation for filename usage

docs/sources/reference/components/otelcol/otelcol.auth.bearer.md

@@ -38,7 +38,7 @@ Name     | Type     | Description                                      | Default
 `scheme` | `string` | Authentication scheme name.                      | "Bearer" | no
 
 When sending the token, the value of `scheme` is prepended to the `token` value.
-The string is then sent out as either a header (in case of HTTP) or as metadata (in case of gRPC).
+The string is then sent out as either a header (in case of HTTP) or as metadata (in case of gRPC). If we are using file for storing the token we can refer the value of `token` using `local.file`
 
 ## Blocks
 
@@ -97,7 +97,7 @@ otelcol.auth.bearer "creds" {
 
 The example below configures [otelcol.exporter.otlphttp][] to use a bearer token authentication.
 
-If we assume that the value of the `API_KEY` environment variable is `SECRET_API_KEY`, then 
+If we assume that the value of the `API_KEY` environment variable is `SECRET_API_KEY`, then
 the `Authorization` HTTP header is set to `MyScheme SECRET_API_KEY`.
 
 ```alloy