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

Documentation/bpf/btf.rst

@@ -369,7 +369,8 @@ No additional type data follow ``btf_type``.
   * ``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``.
@@ -380,6 +381,9 @@ 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~