net: filter: doc: expand and improve BPF documentation

In particular, this patch tries to clarify internal BPF calling
convention and adds internal BPF examples, JIT guide, use cases.

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>