manpage: gnt-group remove cannot remove last group

Signed-off-by: Stephen Shirley <diamond@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

man/gnt-group.rst

@@ -86,7 +86,8 @@ REMOVE
 
 | **remove** {*group*}
 
-Deletes the indicated node group, which must be empty.
+Deletes the indicated node group, which must be empty. There must always be at
+least one group, so the last group cannot be removed.
 
 LIST
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