bpf, docs: Better document the legacy packet access instruction

Use consistent terminology and structured RST elements to better document
these two oddball instructions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220131183638.3934982-4-hch@lst.de