docs/bpf: Update documentation for BTF_KIND_FUNC

The vlen bits in the BTF type of kind BTF_KIND_FUNC are used to convey the
linkage information for functions. The Linux kernel only supports
linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL at this time.

Signed-off-by: Indu Bhagat <indu.bhagat@oracle.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/20220714223310.1140097-1-indu.bhagat@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>