nixos/kernel: link manual (with backport policies) from boot.kernelPackages

Yes I know it's dirty to link the rendered HTML, but linking
`#sec-kernel-config` breaks the manual build for me with the following
error:

    element link: validity error : IDREF attribute linkend references an unknown ID "sec-kernel-config"

This is something that should probably be fixed, but for the current
change I'd argue that this is good enough (in fact even the
`nix.settings`-option uses that hack).

nixos/modules/system/boot/kernel.nix

@@ -62,6 +62,11 @@ in
         configuration.  For instance, if you use the NVIDIA X driver,
         then it also needs to contain an attribute
         {var}`nvidia_x11`.
+
+        Please note that we strictly support kernel versions that are
+        maintained by the Linux developers only. More information on the
+        availability of kernel versions is documented
+        [in the Linux section of the manual](https://nixos.org/manual/nixos/unstable/index.html#sec-kernel-config).
       '';
     };