Presence modifications from MatrixPresence notes.
Presence modifications from MatrixPresence notes.

- Idle is no longer a flag but a full state like online|offline.
- Clarify status examples.
- Add device ID to presence state API given it is per device.
- Add note on device specificity on event stream presence parameter.
- Add global appear-offline API.

matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc