docs/bpf: Improve documentation for cpu=v4 instructions

Improve documentation for cpu=v4 instructions based on
David's suggestions.

Cc: bpf@ietf.org
Suggested-by: David Vernet <void@manifault.com>
Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728225105.919595-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>