qapi-schema: fix typos and explain 'spice' auth

# @connection-id: SPICE connection id number.  All channels with the same id

#                 belong to the same SPICE session.

#

# @connection-type: SPICE channel type number.  "1" is the main control channel,

#                   filter for this one if you want track spice sessions only

# @connection-type: SPICE channel type number.  "1" is the main control

#                   channel, filter for this one if you want to track spice

#                   sessions only

#

# @channel-id: SPICE channel ID number.  Usually "0", might be different needed

#              when multiple channels of the same type exist, such as multiple

# @channel-id: SPICE channel ID number.  Usually "0", might be different when

#              multiple channels of the same type exist, such as multiple

#              display channels in a multihead setup

#

# @tls: true if the channel is encrypted, false otherwise.

#

# @auth: #optional the current authentication type used by the server

#        'none'  if no authentication is being used

#        'spice' (TODO: describe)

#        'spice' uses SASL or direct TLS authentication, depending on command

#                line options

#

# @channels: a list of @SpiceChannel for each active spice channel

#