Update boot option documentation

Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-options.hx

@@ -219,11 +219,30 @@ Use 'file' as a parallel flash image.
 ETEXI
 
 DEF("boot", HAS_ARG, QEMU_OPTION_boot,
-    "-boot [a|c|d|n] boot on floppy (a), hard disk (c), CD-ROM (d), or network (n)\n")
+    "-boot [order=drives][,once=drives][,menu=on|off]\n"
+    "                'drives': floppy (a), hard disk (c), CD-ROM (d), network (n)\n")
 STEXI
-@item -boot [a|c|d|n]
-Boot on floppy (a), hard disk (c), CD-ROM (d), or Etherboot (n). Hard disk boot
-is the default.
+@item -boot [order=@var{drives}][,once=@var{drives}][,menu=on|off]
+
+Specify boot order @var{drives} as a string of drive letters. Valid
+drive letters depend on the target achitecture. The x86 PC uses: a, b
+(floppy 1 and 2), c (first hard disk), d (first CD-ROM), n-p (Etherboot
+from network adapter 1-4), hard disk boot is the default. To apply a
+particular boot order only on the first startup, specify it via
+@option{once}.
+
+Interactive boot menus/prompts can be enabled via @option{menu=on} as far
+as firmware/BIOS supports them. The default is non-interactive boot.
+
+@example
+# try to boot from network first, then from hard disk
+qemu -boot order=nc
+# boot from CD-ROM first, switch back to default order after reboot
+qemu -boot once=d
+@end example
+
+Note: The legacy format '-boot @var{drives}' is still supported but its
+use is discouraged as it may be removed from future versions.
 ETEXI
 
 DEF("snapshot", 0, QEMU_OPTION_snapshot,