alternative of alternatives vs invalidation

[reschke]

From @hruellan in https://lists.w3.org/Archives/Public/ietf-http-wg/2015OctDec/0375.html:

In 2.4, the third paragraphs says that a client can become aware of
multiple alternative services, citing: "or, an alternative service might
itself advertise an alternative".

However, due to the invalidation rule, if an alternative service
advertise an alternative, this new alternative invalidates the previous
list of alternatives.
So if you have Origin -> alt1 -> alt2, then you keep only alt2 and don't
have a list.
I therefore propose to remove the "or, an alternative service might
itself advertise an alternative".