minor fix to build doc (#2907)

Co-authored-by: Henry Li <lihl@microsoft.com>

toolkit/docs/building/building.md

@@ -209,7 +209,7 @@ sudo make iso CONFIG_FILE=./imageconfigs/full.json REBUILD_TOOLS=y
 To create an unattended ISO installer (no interactive UI) use `UNATTENDED_INSTALLER=y` and run with a [`CONFIG_FILE`](https://github.com/microsoft/CBL-MarinerDemo#image-config-file) that only specifies a _single_ SystemConfig.
 
 ```bash
-# Build the standard ISO with unattended installer that installs onto the default Gen1 HyperV VM
+# Build the standard ISO with unattended installer that installs onto the default Gen1 HyperV VM. Needs to cloud-init provision the user once unattended installation finishes.
 sudo make iso -j$(nproc) CONFIG_FILE=./imageconfigs/core-legacy-unattended-hyperv.json REBUILD_TOOLS=y UNATTENDED_INSTALLER=y
 ```