docs: clarify AUTOINSTALL is currently supported on Grml live systems…

… only

It could be supported by any live system, but currently only Grml
provides it, so clarify that.

grml-debootstrap.8.txt

@@ -298,9 +298,9 @@ If environment variable AUTOINSTALL is set grml-debootstrap can be executed in a
 full automatic mode. While this mode isn't really useful for interactive
 execution (just configure /etc/debootstrap/configure or specify the relevant
 variables on the command line instead) it is meant for use via boot option
-debian2hd. The boot option debian2hd (more precise: the kernelname on bootprompt)
-supports the following boot options (they correspond with the command line options
-mentioned above):
+debian2hd on the Grml live system. The boot option debian2hd (more precise: the
+kernelname on bootprompt) supports the following boot options (they correspond
+with the command line options mentioned above):
 
   target=...