docs/bpf: Add KF_RCU documentation

Add proper KF_RCU documentation in kfuncs.rst.

Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221203184613.478967-1-yhs@fb.com


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