Add documentation for Multiboot

The documentation shows how to use -kernel and friends for booting Linux,
but obviously knows nothing about multiboot yet.

Let's include some documentation for multiboot, so people know how to fully
exploit this cool new feature.
Signed-off-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-options.hx

@@ -1100,10 +1100,11 @@ ETEXI
 
 DEFHEADING()
 
-DEFHEADING(Linux boot specific:)
+DEFHEADING(Linux/Multiboot boot specific:)
 STEXI
-When using these options, you can use a given
-Linux kernel without installing it in the disk image. It can be useful
+
+When using these options, you can use a given Linux or Multiboot
+kernel without installing it in the disk image. It can be useful
 for easier testing of various kernels.
 
 @table @option
@@ -1113,7 +1114,8 @@ DEF("kernel", HAS_ARG, QEMU_OPTION_kernel, \
     "-kernel bzImage use 'bzImage' as kernel image\n")
 STEXI
 @item -kernel @var{bzImage}
-Use @var{bzImage} as kernel image.
+Use @var{bzImage} as kernel image. The kernel can be either a Linux kernel
+or in multiboot format.
 ETEXI
 
 DEF("append", HAS_ARG, QEMU_OPTION_append, \
@@ -1128,6 +1130,13 @@ DEF("initrd", HAS_ARG, QEMU_OPTION_initrd, \
 STEXI
 @item -initrd @var{file}
 Use @var{file} as initial ram disk.
+
+@item -initrd "@var{file1} arg=foo,@var{file2}"
+
+This syntax is only available with multiboot.
+
+Use @var{file1} and @var{file2} as modules and pass arg=foo as parameter to the
+first module.
 ETEXI
 
 STEXI