_resolve_room_identity() classifies any room with <=2 joined members as a DM
regardless of m.direct or an explicit room name, so those rooms silently
bypass MATRIX_ALLOWED_ROOMS, MATRIX_FREE_RESPONSE_ROOMS, and
MATRIX_REQUIRE_MENTION, and use DM threading instead of
MATRIX_AUTO_THREAD/MATRIX_SESSION_SCOPE. This was previously only visible in
an inline code comment, not in the env-var docs an operator would read.
Fixes#114733
f45ed8e4ec