bpf: Add some comments to stack representation

Add comments to the datastructure tracking the stack state, as the
mapping between each stack slot and where its state is stored is not
entirely obvious.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20231208032519.260451-2-andreimatei1@gmail.com