`is_direct` field on `/createRoom` body does not automatically mark the room as a DM

**Link to problem area**: https://spec.matrix.org/v1.9/client-server-api/#creation and https://spec.matrix.org/v1.9/client-server-api/#direct-messaging

**Issue**
I naively expected that passing this flag to /createRoom would automatically mark the created room as a DM in the creator's account data. This seems not to be the case: the referenced bits of the spec mainly talk about propagating the `is_direct` flag to invitees.

(Might be appropriate to close this as WONTFIX if this doesn't bother today's clients, and if canonical DMs will fix it.)

Read the Matrix Client-Server API sections for room creation and direct messaging linked in the issue, focusing on the meaning of is_direct and creator account data. Determine whether the specification should require this behavior or explicitly document that it does not, then update the relevant specification text and any applicable examples or tests.

Written by the indexing model from the issue text.