Clarify that NixOS cfg is needed to make zfs.ko available

The documented configuration changes for NixOS appear to
be making zfs available for boot.  However, these changes
are also required just to make the zfs.ko module available
to modprobe even for users who don't need ZFS available at
boot time.  Also, the kernel module does not appear until
after a reboot, regardless of 'nixos-rebuild switch'.

(a more knowledgable NixOS user might know how to modprobe
 without a reboot, but I don't)

Author: nrdvana

docs/Getting Started/NixOS/index.rst

@@ -30,6 +30,10 @@ see below.
 
 Live image ships with ZFS support by default.
 
+Note that you need to apply these settings even if you don't need
+to boot from ZFS.  The kernel module 'zfs.ko' will not be available
+to modprobe until you make these changes and reboot.
+
 #. Import separate configration file for ZFS options::
 
     vim /etc/nixos/configuration.nix