Update boot option documentation

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,