discuss RP ID and how the RP may declare it such that creds are valid for subdomains

[equalsJeffH]

cf. https://w3c.github.io/webauthn/#relying-party-identifier

<someone> reports:
"Going over stack overflow webauthn posts I found two people running into this: they have two websites at domain.com & subdomain.domain.com. They register a key at domain.com and are surprised that it doesn't work on get assertion for subdomain.domain.com. The reason is they are not setting an RP ID of domain.com, instead leaving the default (which is the origin)."