Note btrfs+SELinux support issue in docs

I've added sections to the Fedora/CentOS/RHEL docs that document the [current incompatibility](https://bugzilla.redhat.com/show_bug.cgi?id=1128041) of btrfs and SELinux with Docker. I picked those three distros because I've observed/documented this behavior on Fedora (#7952), issue #7318 documented it on CentOS and the RHEL Bugzilla linked above implies it's present for RHEL.