Reiterate handling of unknown mandatory SVCB parameters

[tfpauly]

We should clarify that if a client doesn’t understand a mandatory parameter in an SVCB record, it MUST NOT use it as normal DoT/DoH, since it could be misinterpreting the meaning.

[enygren]

Concrete example might be with:

"mandatory=foo,dohpath foo=[...] alpn=h2 dohpath=[...]"

Where clients which don't know how to handle "foo" must ignore that RR in the RRset, which might also contain other SVCB RRs for just DoH.