man/ukify: mention all functionality in intro, add example of direct boot

Over the time, the functionality in ukify has grown. This should all be briefly
mentioned in the first section so the user does't have to read the whole page
to figure out what types of functionality are implemnted.

Also add an example of direct kernel boot. It's a nifty technology (and frankly
underutilized, considering how cool it is is).

man/ukify.xml

@@ -45,11 +45,19 @@
     <title>Description</title>
 
     <para><command>ukify</command> is a tool whose primary purpose is to combine components (usually a
-    kernel, an initrd, and a UEFI boot stub) to create a
-    <ulink url="https://uapi-group.org/specifications/specs/unified_kernel_image/">Unified Kernel Image (UKI)</ulink>
-    — a PE binary that can be executed by the firmware to start the embedded linux kernel.
-    See <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>
-    for details about the stub.</para>
+    kernel, an initrd, and the
+    <citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry> UEFI
+    stub) to create a <ulink url="https://uapi-group.org/specifications/specs/unified_kernel_image/">Unified
+    Kernel Image (UKI)</ulink> — a single PE binary that boots the system. When the UKI is executed, the stub
+    extracts and boots the embedded linux kernel. The UKI can be started directly by the firmware or through
+    a boot loader. When used with <ulink url="https://www.qemu.org/docs/master/">qemu</ulink>, a UKI can also
+    be executed through "direct kernel boot", see example below.</para>
+
+    <para><command>ukify</command> can also be used generate other types of UKI-like images, in particular
+    extensions. See the description of the <command>build</command> verb below. <command>ukify</command> can
+    also generate certificates and keys for SecureBoot and PCR signing, see the description of the
+    <command>genkey</command> verb below. <command>ukify</command> can also print detailed information about
+    unified kernel images, see the description of <command>inspect</command> verb below.</para>
   </refsect1>
 
   <refsect1>
@@ -719,6 +727,27 @@
       <para>This creates an unsigned UKI <filename>./vmlinuz.unsigned.efi</filename>.</para>
     </example>
 
+    <example>
+      <title>Direct kernel boot in a virtual machine</title>
+
+      <para>When using <ulink url="https://www.qemu.org/docs/master/">qemu</ulink> with
+      <ulink url="https://www.linux-kvm.org/downloads/lersek/ovmf-whitepaper-c770f8c.txt">OVMF</ulink>
+      (UEFI Firmware for virtual machines) the <option>-kernel</option> switch can be used directly with a
+      UKI. Example:</para>
+
+      <para>
+        <command>qemu-kvm
+        -drive if=pflash,format=qcow2,readonly=on,file=/usr/share/edk2/ovmf/OVMF_CODE_4M.qcow2
+        -kernel <filename index='false'>./vmlinuz.unsigned.efi</filename>
+        <replaceable>[ ... ]</replaceable>
+        </command>
+      </para>
+
+      <para>(The path to the firmware file might need to be adjusted depending on the distribution.) Usually,
+      another <option>-drive</option> argument would to be used to attach an actual disk image, but this
+      is not required.</para>
+    </example>
+
     <example>
       <title>All the bells and whistles</title>