bpf, docs: Better document the regular load and store instructions

Add a separate section and a little intro blurb for the regular load and
store instructions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220131183638.3934982-3-hch@lst.de