bpf, docs: Add subsections for ALU and JMP instructions

Add a little more stucture to the ALU/JMP documentation with sections and
improve the example text.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220103183556.41040-3-hch@lst.de