session: update spice_session_connect() doc
session: update spice_session_connect() doc

Update the documentation about the return value, and how to watch for
connection success.

Releated to:
https://bugzilla.redhat.com/show_bug.cgi?id=1253848

fgouget/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk