bpf, doc: Document Jump X addressing mode

bpf_asm and the other classic BPF tools support jump conditions
comparing register A to register X, in addition to comparing
register A with constant K.

Only the latter was documented in filter.txt, add two new addressing
modes that describe the former.

Signed-off-by: Arthur Fabre <arthur@arthurfabre.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>