In commit da703149 ("bpf: Document BPF_PROG_TYPE_CGROUP_SYSCTL")
Andrey proposes to put per-prog type docs under Documentation/bpf/

Let's move flow dissector documentation there as well.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>