bpf, docs: Add a setion to explain the basic instruction encoding

The eBPF instruction set document does not currently document the basic
instruction encoding.  Add a section to do that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220103183556.41040-2-hch@lst.de