GTlsClientConnection: loosen the semantics of "use-ssl3"

If SSL 3.0 has been disabled (at the host, application, or library
level), then the "use-ssl3" property becomes a "fail-immediately"
property.

Despite the name, the point of the property wasn't really specifically
to use SSL 3.0; it was to allow fallback when talking to broken
servers that do SSL/TLS negotiation incorrectly and break when they
see unexpectedly-high version numbers. So if we can't fall back to SSL
3.0, then the "use-ssl3" property should fall back to TLS 1.0 instead
(since there are hosts that will reject a TLS 1.2 handshake, but
accept a TLS 1.0 one).

glib-networking is being updated to implement that behavior, so update
the documentation here.

https://bugzilla.gnome.org/show_bug.cgi?id=738633