update CLOUDFLARE_DURABLE_OBJECT_ID env var description

Author: emily-shen

src/content/docs/containers/platform-details/environment-variables.mdx

@@ -15,7 +15,7 @@ The container runtime automatically sets the following variables:
 - `CLOUDFLARE_COUNTRY_A2` - the [ISO 3166-1 Alpha 2 code](https://www.iso.org/obp/ui/#search/code/) of a country the container is placed in
 - `CLOUDFLARE_LOCATION` - a name of a location the container is placed in
 - `CLOUDFLARE_REGION` - a region name
-- `CLOUDFLARE_DURABLE_OBJECT_ID` - the ID of the Durable Object instance that the container is bound to
+- `CLOUDFLARE_DURABLE_OBJECT_ID` - the ID of the Durable Object instance that the container is bound to. You can use this to identify particular container instances on the dashboard.
 
 ## User-defined environment variables