bpf, docs: Add table captions

As suggested by Ines Robles in his IETF GENART review at
https://datatracker.ietf.org/doc/review-ietf-bpf-isa-02-genart-lc-robles-2024-05-16/

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Link: https://lore.kernel.org/r/20240524164618.18894-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>