vnc: Clarify documentation of QMP command change

QMP command

    { "execute": "change",
      "arguments": { "device": "vnc", "target": "password", "arg": PWD } }

behaves just like

    { "execute": "change-vnc-password",
      "arguments": { "password", "arg": PWD } }

Their documentation differs, however.  According to
change-vnc-password's documentation, "an empty password [...] will set
the password to the empty string", while change's documentation claims
"no future logins will be allowed".  The former is actually correct.
Replace the incorrect claim by a reference to change-vnc-password.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 1500448182-21376-1-git-send-email-armbru@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
