Documentation: Describe bpf reference tracking

Document the new pointer types in the verifier and how the pointer ID
tracking works to ensure that references which are taken are later
released.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>