docs/manual: document makeusers group-only creation

[Thomas: slightly reword according to the help text.]

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/makeusers-syntax.txt

@@ -16,7 +16,8 @@ user per line; the fields are:
 Where:
 
 - +username+ is the desired user name (aka login name) for the user.
-  It can not be +root+, and must be unique.
+  It can not be +root+, and must be unique. If set to +-+, then just a
+  group will be created.
 - +uid+ is the desired UID for the user. It must be unique, and not
   +0+. If set to +-1+, then a unique UID will be computed by Buildroot
   in the range [1000...1999]