qemu-doc: mark ppc/prep machine as deprecated

40p machine type should be used instead.
Signed-off-by: NHervé Poussineau <hpoussin@reactos.org>
Acked-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

qemu-deprecated.texi

@@ -195,6 +195,12 @@ support page sizes < 4096 any longer.
 These machine types are very old and likely can not be used for live migration
 from old QEMU versions anymore. A newer machine type should be used instead.
 
+@subsection prep (PowerPC) (since 3.1)
+
+This machine type uses an unmaintained firmware, broken in lots of ways,
+and unable to start post-2004 operating systems. 40p machine type should be
+used instead.
+
 @section Device options
 
 @subsection Block device options