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

Documentation/bpf/kfuncs.rst

@@ -191,6 +191,15 @@ rebooting or panicking. Due to this additional restrictions apply to these
 calls. At the moment they only require CAP_SYS_BOOT capability, but more can be
 added later.
 
+2.4.8 KF_RCU flag
+-----------------
+
+The KF_RCU flag is used for kfuncs which have a rcu ptr as its argument.
+When used together with KF_ACQUIRE, it indicates the kfunc should have a
+single argument which must be a trusted argument or a MEM_RCU pointer.
+The argument may have reference count of 0 and the kfunc must take this
+into consideration.
+
 2.5 Registering the kfuncs
 --------------------------