Clarify rooms.md
Clarify rooms.md

Co-authored-by: Patrick Cloke <7509937bf24a07592c6933b8e1ea5db7c1f5f52b@users.noreply.github.com>
matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse