veristat: Document verifier log dumping capability

`-vl2` is a useful combination of flags to dump the entire
verification log. This is helpful when making changes to the verifier,
as you can see what it thinks program one instruction at a time.

This was more or less a hidden feature before. Document it so others can
discover it.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/d57bbcca81e06ae8dcdadaedb99a48dced67e422.1736466129.git.dxu@dxuuu.xyz