doc - enable environment variable in configuration (#3385)

Environment variable in configuration works only if promtail is started with `-config.expand-env=true` option.
Should we add `Note: This feature is only available in Loki 2.1+.` too?

docs/sources/clients/promtail/configuration.md

@@ -84,7 +84,7 @@ from scraped targets, see [Pipelines](../pipelines/).
 ### Use environment variables in the configuration
 
 You can use environment variable references in the configuration file to set values that need to be configurable during deployment.
-To do this, use:
+To do this, pass `-config.expand-env=true` and use:
 
 ```
 ${VAR}