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 <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

Documentation/bpf/btf.rst

···
   * ``name_off``: offset to a valid C identifier
   * ``info.kind_flag``: 0
   * ``info.kind``: BTF_KIND_FUNC
-  * ``info.vlen``: 0
+  * ``info.vlen``: linkage information (BTF_FUNC_STATIC, BTF_FUNC_GLOBAL
+                   or BTF_FUNC_EXTERN)
   * ``type``: a BTF_KIND_FUNC_PROTO type
 
 No additional type data follow ``btf_type``.
···
 type. The BTF_KIND_FUNC may in turn be referenced by a func_info in the
 :ref:`BTF_Ext_Section` (ELF) or in the arguments to :ref:`BPF_Prog_Load`
 (ABI).
+
+Currently, only linkage values of BTF_FUNC_STATIC and BTF_FUNC_GLOBAL are
+supported in the kernel.
 
 2.2.13 BTF_KIND_FUNC_PROTO
 ~~~~~~~~~~~~~~~~~~~~~~~~~~