1128329 - Add warnings about admin.conf to docs.

admin.conf gets overridden in the upgrade from 2.3.1 to 2.4.0. This
commit adds warnings to our release notes so that users know to backup
admin.conf before they upgrade if they wish to keep their settings.

https://bugzilla.redhat.com/show_bug.cgi?id=1128329
I believe /etc/pulp/admin/admin.conf should be escaped with double backicks. I believe that's how all conf file references are written.

``/etc/pulp/admin/admin.conf``