add info about .dev.vars.<environment-name> (#17594)

Author: emily-shen

src/content/partials/workers/secrets-in-dev.mdx

@@ -11,3 +11,5 @@ The `.dev.vars` file should be formatted like a `dotenv` file, such as `KEY="VAL
 SECRET_KEY="value"
 API_TOKEN="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9"
 ```
+
+You can set secrets per environment by creating additional files with the naming convention `.dev.vars.<environment-name>`. Like other environment variables, secrets are [non-inheritable](/workers/wrangler/configuration/#non-inheritable-keys) and must be defined per environment.