feat: Transfer the broadcast secret in an encrypted message rather than directly in the QR code

src/param.rs

@@ -103,6 +103,9 @@ pub enum Param {
     /// removed from the group.
     ///
     /// For "MemberAddedToGroup" this is the email address added to the group.
+    ///
+    /// For securejoin messages, this is the step,
+    /// which is put into the `Secure-Join` header.
     Arg = b'E',
 
     /// For Messages
@@ -111,6 +114,9 @@ pub enum Param {
     ///
     /// For `BobHandshakeMsg::RequestWithAuth`, this is the `Secure-Join-Auth` header.
     ///
+    /// For version two of the securejoin protocol (`vb-request-v2`),
+    /// this is the Auth token used to encrypt the message.
+    ///
     /// For [`SystemMessage::MultiDeviceSync`], this contains the ids that are synced.
     ///
     /// For [`SystemMessage::MemberAddedToGroup`],