Add clearer explanation about hostname being ignored in GAMMA

[NaturezzZ]

What would you like to be added:

Explain in API specification that hostnames are ignored in GAMMA, which is described in GEP1426.

Why this is needed:

It's frustrated for a user to find that hostnames being ignored is not noted in the specification of gateway-api. This important information is only described in gateway enhancement proposal. Interestingly, istio and linkerd have different behavior on whether to ignore hostnames in GAMMA implementation.