manual: more warnings to not use output/target

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/using.txt

@@ -86,7 +86,8 @@ This directory contains several subdirectories:
 * +target/+ which contains 'almost' the complete root filesystem for
   the target: everything needed is present except the device files in
   +/dev/+ (Buildroot can't create them because Buildroot doesn't run
-  as root and doesn't want to run as root). Therefore, this directory
+  as root and doesn't want to run as root). Also, it doesn't have the correct
+  permissions (e.g. setuid for the busybox binary). Therefore, this directory
   *should not be used on your target*.  Instead, you should use one of
   the images built in the +images/+ directory. If you need an
   extracted image of the root filesystem for booting over NFS, then