Add documentation about mock changes (#1473880)

mock now uses systemd-nspawn by default, but it cannot setup the
/dev/loop* nodes that are needed by lorax and livemedia-creator so users
will need to pass --old-chroot to mock if they are using it.

docs/livemedia-creator.rst

@@ -359,8 +359,9 @@ As of lorax version 22.2 you can use livemedia-creator and anaconda version
 22.15 inside of a mock chroot with --make-iso and --make-fsimage.
 
 .. note::
-    As of mock 1.2.12 you no longer need to bind mount ``/dev/``, loop devices are setup
+    As of mock 1.3.4 you need to use ``--old-chroot`` with mock. Mock now defaults to using systemd-nspawn
-    as part of the standard mock ``/dev/`` creation.
+    which cannot create the needed loop device nodes. Passing ``--old-chroot`` will use the old system
+    where ``/dev/loop*`` is setup for you.
 
 On the host system:
 

docs/lorax.rst

@@ -44,6 +44,15 @@ Under ``./results/`` will be the release tree files: .discinfo, .treeinfo, every
 goes onto the boot.iso, the pxeboot directory, and the boot.iso under ``./images/``.
 
 
+Running inside of mock
+----------------------
+
+If you are using lorax with mock v1.3.4 or later you will need to pass
+``--old-chroot`` to mock. Mock now defaults to using systemd-nspawn which cannot
+create the needed loop device nodes. Passing ``--old-chroot`` will use the old
+system where ``/dev/loop*`` is setup for you.
+
+
 How it works
 ------------