doc: clarify warning message for rootfs setup

Signed-off-by: Yılmaz ŞEN <yilmazz_senn@hotmail.com>

docs/rootfs-and-kernel-setup.md

@@ -119,9 +119,15 @@ Alpine Linux:
 
    # Then, copy the newly configured system to the rootfs image:
    for d in bin etc lib root sbin usr; do tar c "/$d" | tar x -C /my-rootfs; done
+
+   # The above command may trigger the following message:
+   # tar: Removing leading "/" from member names
+   # However, this is just a warning, so you should be able to
+   # proceed with the setup process.
+
    for dir in dev proc run sys var; do mkdir /my-rootfs/${dir}; done
 
-   # All done, exit docker shell
+   # All done, exit docker shell.
    exit
    ```