WIP clarifications

server-protocol.md

@@ -234,7 +234,10 @@ coming from the other client.
 Each mailbox is "opened" by some number of clients at a time, until all
 clients have closed it. Mailboxes are kept alive by either an open client, or
 a Nameplate which points to the mailbox (so when a Nameplate is deleted from
-inactivity, the corresponding Mailbox will be too).
+inactivity, the corresponding Mailbox will be too). The mailbox server makes
+sure a mailbox can never be opened by more than two parties, neither
+simultaneously not cumulatively. Trying to open a mailbox as a third party
+results in a "crowded" error for that attempt.
 
 The `open` command both marks the mailbox as being opened by the bound side,
 and also adds the WebSocket as subscribed to that mailbox, so new messages
@@ -292,7 +295,9 @@ continue to send messages (so-called "half-closed state") until sending a `close
 message too, which in turn will send the `closed` command to the first client and
 thus finalize the session teardown. The second client may disconnect directly
 after sending the `close` message, there will be no further confirmation. It is
-an error to send messages after having sent `close`.
+an error to send messages after having sent `close`. If a client closes the
+mailbox before the other side opens it, the second client will get a "closed"
+error message while trying to open the mailbox.
 
 ## All Message Types