Clarify "is a valid event" requirement in PDU checks

[tulir]

Link to problem area: https://spec.matrix.org/unstable/server-server-api/#checks-performed-on-receipt-of-a-pdu

Issue: The first check, "Is a valid event, otherwise it is dropped.", could be interpreted as validating the entire event, including the content. Only validating the top-level keys seems more reasonable (related: #2801), but either way it should be specified explicitly.

Context: Conduit interpreted the requirement as validating the content, and is validating the content of m.room.create events, which leads to being unable to join rooms where the predecessor doesn't contain a valid event ID (there's no explicitly defined validation of predecessor in auth rules or other such places)

[richvdh]

I think this is a dup of matrix-org/matrix-spec#365?

[tulir]

I guess yeah, but there has been no progress on that issue 😿