bpf, docs: Move the packet access instructions last in instruction-set.rst

The packet access instructions are a convoluted leftover from classic
BPF.  Move them last past the much more important atomic operations,
and improve the rendering of the code example.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211223101906.977624-5-hch@lst.de