net: filter: document internal instruction encoding

This patch adds a description of eBPFs instruction encoding in order
to bring the documentation in line with the implementation.

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>