Add a note for setting syntax

guides/administrator-guides/settings-via-env-vars.md

@@ -4,14 +4,16 @@ You can set any settings value via an environment variable when starting your se
 
 ## Set Setting
 
-To set a setting you need only to add the env var `SETTING_ID=VALUE` when starting the server.
+To set a setting for the first time you need only to add the env var `SETTING_ID=VALUE` when starting the server.
 
 Example:
 
 ```text
 FileUpload_Storage_Type=GoogleCloudStorage
 ```
 
+**Note: This only works if the setting isn't already set.  So this is most useful for setting on initial start.  If the setting already has a value you will need to use the syntax below**
+
 ## Overwrite Setting
 
 You can overwrite any setting by setting an environment variable prefixed with: `OVERWRITE_SETTING_`