docs: clarify UEFI bootloader setup

Changed files
+2 -1
nixos
doc
manual
installation
+2 -1
nixos/doc/manual/installation/installing-uefi.xml
···
<literal>vfat</literal> filesystem.</para>
</listitem>
<listitem>
-
<para>You must set <option>boot.loader.systemd-boot.enable</option> to
+
<para>Instead of <option>boot.loader.grub.device</option>,
+
you must set <option>boot.loader.systemd-boot.enable</option> to
<literal>true</literal>. <command>nixos-generate-config</command>
should do this automatically for new configurations when booted in
UEFI mode.</para>