input: improve docs for input-send-event qmp command

# Send input event(s) to guest.

#

# @console: #optional console to send event(s) to.

#           This parameter can be used to send the input event to

#           specific input devices in case (a) multiple input devices

#           of the same kind are added to the virtual machine and (b)

#           you have configured input routing (see docs/multiseat.txt)

#           for those input devices.  If input routing is not

#           configured this parameter has no effect.

#           If @console is missing, only devices that aren't associated

#           with a console are admissible.

#           If @console is specified, it must exist, and both devices

#           associated with that console and devices not associated with a

#           console are admissible, but the former take precedence.

#

# @events: List of InputEvent union.

#