bpf: Add comment about helper freeze

Put a comment after the bpf helper list in uapi bpf.h to prevent people
from trying to add new helpers there and direct them to kfuncs.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Levi Zim <rsworktech@outlook.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/bpf/CAEf4BzZvQF+QQ=oip4vdz5A=9bd+OmN-CXk5YARYieaipK9s+A@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20221231004213.h5fx3loccbs5hyzu@macbook-pro-6.dhcp.thefacebook.com/
Link: https://lore.kernel.org/bpf/20250204-bpf-helper-freeze-v1-1-46efd9ff20dc@outlook.com