Add note about MSC3886, MSC3903 and MSC3906 being closed (#4189)

* Add note about MSC3886, MSC3903 and MSC3906 being closed

* Move comments in to jsdoc

---------

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/rendezvous/MSC3906Rendezvous.ts

@@ -59,6 +59,9 @@ const LOGIN_TOKEN_PROTOCOL = new UnstableValue("login_token", "org.matrix.msc390
  * Implements MSC3906 to allow a user to sign in on a new device using QR code.
  * This implementation only supports generating a QR code on a device that is already signed in.
  * Note that this is UNSTABLE and may have breaking changes without notice.
+ * MSC3886/MSC3903/MSC3906 are now closed and so this functionality will be removed in future.
+ * However, we want to keep this implementation around for some time.
+ * TODO: define an end-of-life date for this implementation.
  */
 export class MSC3906Rendezvous {
     private newDeviceId?: string;

src/rendezvous/channels/MSC3903ECDHv2RendezvousChannel.ts

@@ -69,6 +69,9 @@ async function importKey(key: Uint8Array): Promise<CryptoKey> {
  * Implementation of the unstable [MSC3903](https://github.com/matrix-org/matrix-spec-proposals/pull/3903)
  * X25519/ECDH key agreement based secure rendezvous channel.
  * Note that this is UNSTABLE and may have breaking changes without notice.
+ * MSC3886/MSC3903/MSC3906 are now closed and so this functionality will be removed in future.
+ * However, we want to keep this implementation around for some time.
+ * TODO: define an end-of-life date for this implementation.
  */
 export class MSC3903ECDHv2RendezvousChannel<T> implements RendezvousChannel<T> {
     private olmSAS?: SAS;

src/rendezvous/transports/MSC3886SimpleHttpRendezvousTransport.ts

@@ -37,6 +37,9 @@ export interface MSC3886SimpleHttpRendezvousTransportDetails extends RendezvousT
  * Implementation of the unstable [MSC3886](https://github.com/matrix-org/matrix-spec-proposals/pull/3886)
  * simple HTTP rendezvous protocol.
  * Note that this is UNSTABLE and may have breaking changes without notice.
+ * MSC3886/MSC3903/MSC3906 are now closed and so this functionality will be removed in future.
+ * However, we want to keep this implementation around for some time.
+ * TODO: define an end-of-life date for this implementation.
  */
 export class MSC3886SimpleHttpRendezvousTransport<T extends {}> implements RendezvousTransport<T> {
     private uri?: string;