clarify invalidation rules wrt altsvc frame

[reschke]

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

Section 3.1 defines the rule for invalidation. However, in my sense it
is not crystal clear that it applies also to alternatives discovered
through the ALTSVC frame.
I'd suggest adding that the ALTSVC frame is just another way of
transmitting an Alt-SVC header field, and therefore all the rules
applying to the header field also apply to the frame.
We may want to go a step further and say the rules defined for the
Alt-SVC header field apply to any other possibility of discovering
alternative services (including those not supported).