add hardware device descriptions to doc

most of the text is recycled and follows no particular format

devices/qemu/default.nix

@@ -12,6 +12,33 @@
     };
   };
 
+  description = ''
+    QEMU
+    ****
+
+    This is not a hardware device. This target produces an image for
+    QEMU, the "generic and open source machine emulator and
+    virtualizer".
+
+    Liminix can build QEMU for both MIPS (:code:`qemu` device) and Aarch64 (:code:`qemu-aarch64` device)
+
+    MIPS QEMU emulates a "Malta" board, which was an ATX form factor
+    evaluation board made by MIPS Technologies, but mostly in Liminix
+    we use paravirtualized devices (Virtio) instead of emulating
+    hardware. For Aarch64 we use the QEMU "virt" board.
+
+    Building an image for QEMU results in a :file:`result/` directory
+    containing ``run.sh`` ``vmlinux``, ``rootfs`` and possibly
+    (architecture-dependent) ``Image``. To invoke the emulator,
+    run ``run.sh``.
+
+    The configuration includes two emulated "hardware" ethernet
+    devices and the kernel :code:`mac80211_hwsim` module to
+    provide an emulated wlan device. To read more about how
+    to connect to this network, refer to :ref:`qemu-networking`
+    in the Development manual.
+
+  '';
   module = {pkgs, config, ... }: {
     imports = [ ../../modules/arch/mipseb.nix ];
     kernel = {