docs(readme.rst): add doc for apt_conf state + other minor update

docs/README.rst

 Check https://wiki.debian.org/SourcesList for an explanation about the resulting
 files structure.
 
+``apt.apt_conf``
+^^^^^^^^^^^^^^^^^^^
+
+Allows you to manage apt configuration from pillar. Check ``pillar.example`` to see
+possible values.
+
+Check https://wiki.debian.org/AptConfiguration#apt.conf and ``man 5 apt.conf`` for
+an explanation about the resulting files structure.
+
 ``apt.preferences``
 ^^^^^^^^^^^^^^^^^^^
 
 Allows you to configure and manage apt's preferences from pillar. Check
-``pillar.example`` to see possible values. If used and no repositories are
-provided, sane default values from ``map.jinja`` are used.
+``pillar.example`` to see possible values.
 
-Check https://wiki.debian.org/AptPreferences?action=show&redirect=preferences
+Check https://wiki.debian.org/AptConfiguration#apt_preferences_.28APT_pinning.29
 and ``man 5 apt_preferences`` for an explanation about the resulting files structure.
 
 ``apt.ppa``
 ^^^^^^^^^^^^^^^^^^^^^
 
 Gives you SSH access to the instance for manual testing.
-

pillar.example

   # Set the right keyring for the distro (ie ubuntu-keyring or ...)
   keyring_package: debian-archive-keyring
 
+  remove_apt_conf: true
+  clean_apt_conf_d: true
+
   remove_sources_list: true
   clean_sources_list_d: true