add new env var info

docs/recipes/environment-variables-system-properties.md

@@ -106,6 +106,13 @@ Specifies the file directory for the Lucee server context.
 **System Property:** `-Dlucee.version`  
 Defines the version of Lucee to load. For example, setting it to `6.1.0.0` will load that version. If not available locally, Lucee will automatically download it from Maven.
 
+
+**Environment Variable:** `LUCEE_ADMIN_MODE="single|multi"`  
+**System Property:** `-Dlucee.admin.mode`  
+This setting only applies to Lucee 6 (above `6.1.1.53`), Lucee 6 can run in `single` mode or `multi` mode. In single mode, Lucee only has one set of configurations for the whole server. In multi mode, you have a base configuration for the whole server, but then every web context has its own configuration to override the base configuration. With Lucee 5, you only have multi mode and with Lucee 7, you only have single mode.
+By default, a new version of Lucee 6 with no `.CFConfig.json` provided that contains a `mode:"single|multi"` setting starts in single mode. When you update from Lucee 5, you start in multi mode.
+To change this default behavior, you can set the environment variable `LUCEE_ADMIN_MODE="single|multi"` or system property `-Dlucee.admin.mode="single|multi"`. However, this setting only influences the default behavior and is overridden by a possible mode setting in `.CFConfig.json`.
+
 ## Breaking Changes
 
 Major updates for Lucee can sometimes cause breaking changes. The settings below allow you to emulate the behavior of older Lucee versions in newer versions.