bpf, docs: Document existing macros instead of deprecated

The BTF_TYPE_SAFE_NESTED macro was replaced by the BTF_TYPE_SAFE_TRUSTED,
BTF_TYPE_SAFE_RCU, and BTF_TYPE_SAFE_RCU_OR_NULL macros. Fix the docs
correspondingly.

Fixes: 6fcd486b ("bpf: Refactor RCU enforcement in the verifier.")
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230622095424.1024244-1-aspsk@isovalent.com