docs/bpf: Update documentation for BTF_KIND_ENUM64 support

Add BTF_KIND_ENUM64 documentation in btf.rst.
Also fixed a typo for section number for BTF_KIND_TYPE_TAG
from 2.2.17 to 2.2.18, and fixed a type size issue for
BTF_KIND_ENUM.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062724.3728215-1-yhs@fb.com


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