Document the changes of vm.unprivileged_userfaultfd in v5.11

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=37cd0575b8510159992d279c530c05f872990b02
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d0d4730ac2e404a5b0da9a87ef38c73e51cb1664

kconfig_hardened_check/__init__.py

@@ -46,6 +46,8 @@
 #    net.core.bpf_jit_harden=2
 #
 #    vm.unprivileged_userfaultfd=0
+#        (at first, it disabled unprivileged userfaultfd,
+#         and since v5.11 it enables unprivileged userfaultfd for user-mode only)
 #
 #    dev.tty.ldisc_autoload=0
 #    fs.protected_symlinks=1