Clarify that this file only lists packages made available

The packages listed here are not automatically installed. The end users
select what to install dynamically through tasksel.

kali-config/installer-default/packages

@@ -4,7 +4,8 @@
 # - kali-linux-everything
 
 # Metapackages
-# You can customize the set of Kali metapackages (groups of tools) to install
+# You can customize the set of Kali metapackages (groups of tools) available
+# in the installer ISO.
 # For the complete list see: https://tools.kali.org/kali-metapackages
 kali-linux-core
 kali-tools-top10