bpf: Document usage of the new __bpf_kfunc macro

Now that the __bpf_kfunc macro has been added to linux/btf.h, include a
blurb about it in the kfuncs.rst file. In order for the macro to
successfully render with .. kernel-doc, we'll also need to add it to the
c_id_attributes array.

Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20230201173016.342758-3-void@manifault.com

Documentation/conf.py

@@ -116,6 +116,9 @@ if major >= 3:
 
             # include/linux/linkage.h:
             "asmlinkage",
+
+            # include/linux/btf.h
+            "__bpf_kfunc",
         ]
 
 else: