Merge branch 'bpf: Update doc about calling kernel function'

Martin KaFai says:

====================

This set updates the document about the bpf program calling kernel
function.  In particular, updates are regarding to the clang
requirement in selftests and kfunc-call not an ABI.
====================

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