webadmin: added SPICE proxy format details to help message

Added this string:
SPICE proxy address format: [protocol://]<host>[:port]. The only supported
protocol is http.

Change-Id: I7b3023edf97c7b065539b4364583669a929510d2
Bug-Url: https://bugzilla.redhat.com/1094026
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>