bpf, docs: Fully document the JMP mode modifiers

Add a description for all the modifiers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220103183556.41040-7-hch@lst.de