docs/bpf: clarify how btf_type_tag gets encoded in the type chain

Clarify where the BTF_KIND_TYPE_TAG gets encoded in the type chain,
so applications and kernel can properly parse them.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220127154627.665163-1-yhs@fb.com


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