Docs: Document the ability to use env vars in ldap.toml (#23304)

This was implemented but never documented.

This is related to:
#8832
#5248
#20173

docs/sources/auth/ldap.md

@@ -92,6 +92,14 @@ member_of = "memberOf"
 email =  "email"
 ```
 
+### Using environment variables
+
+You can interpolate variables in the TOML config from environment variables. For instance, you could externalize your `bind_password` that way:
+
+```bash
+bind_password = "${LDAP_ADMIN_PASSWORD}"
+```
+
 ## LDAP Debug View
 
 > Only available in Grafana v6.4+