filter: doc: improve BPF documentation

This patch significantly updates the BPF documentation and describes
its internal architecture, Linux extensions, and handling of the
kernel's BPF and JIT engine, plus documents how development can be
facilitated with the help of bpf_dbg, bpf_asm, bpf_jit_disasm.
Signed-off-by: NDaniel Borkmann <dborkman@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>