Fix membership state table and diagram (#3730)

* Fix membership state table and diagram

There were 2 missing cases which are legal:
* `invite->knock` (a fairly silly thing to do, but legal under the auth rules)
* `external->leave (via /kick)` (another somewhat silly thing to do, but no different than `external->ban (via /ban)`)

The state table considered the first as illegal, which is untrue.

* Changelog

* Make the graph prettier

* Update changelogs/client_server/newsfragments/3730.clarification

Co-authored-by: Richard van der Hoff <[email protected]>

Co-authored-by: Richard van der Hoff <[email protected]>

Author: turt2live

changelogs/client_server/newsfragments/3730.clarification

@@ -0,0 +1 @@
+Fix membership state transitions to denote that `invite->knock` and `external->leave` are valid transitions.

data/event-schemas/schema/m.room.member.yaml

@@ -31,13 +31,13 @@ description: |-
   from the `prev_content` object on an event. If not present, the user's previous membership must be assumed
   as `leave`.
 
-  |                   | to `invite`          | to `join`                              | to `leave`                                                                                                                              | to `ban`                    | to `knock`         |
-  |-------------------|----------------------|----------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|--------------------|
-  | **from `invite`** | No change.           | User joined the room.                  | If the `state_key` is the same as the `sender`, the user rejected the invite. Otherwise, the `state_key` user had their invite revoked. | User was banned.            | Must never happen. |
-  | **from `join`**   | Must never happen.   | `displayname` or `avatar_url` changed. | If the `state_key` is the same as the `sender`, the user left. Otherwise, the `state_key` user was kicked.                              | User was kicked and banned. | Must never happen. |
-  | **from `leave`**  | New invitation sent. | User joined.                           | No change.                                                                                                                              | User was banned.            | User is knocking.  |
-  | **from `ban`**    | Must never happen.   | Must never happen.                     | User was unbanned.                                                                                                                      | No change.                  | Must never happen. |
-  | **from `knock`**  | Knock accepted.      | Must never happen.                     | If the `state_key` is the same as the `sender`, the user retracted the knock. Otherwise, the `state_key` user had their knock denied.   | User was banned.            | No change.         |
+  |                   | to `invite`          | to `join`                              | to `leave`                                                                                                                              | to `ban`                    | to `knock`           |
+  |-------------------|----------------------|----------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|----------------------|
+  | **from `invite`** | No change.           | User joined the room.                  | If the `state_key` is the same as the `sender`, the user rejected the invite. Otherwise, the `state_key` user had their invite revoked. | User was banned.            | User is re-knocking. |
+  | **from `join`**   | Must never happen.   | `displayname` or `avatar_url` changed. | If the `state_key` is the same as the `sender`, the user left. Otherwise, the `state_key` user was kicked.                              | User was kicked and banned. | Must never happen.   |
+  | **from `leave`**  | New invitation sent. | User joined.                           | No change.                                                                                                                              | User was banned.            | User is knocking.    |
+  | **from `ban`**    | Must never happen.   | Must never happen.                     | User was unbanned.                                                                                                                      | No change.                  | Must never happen.   |
+  | **from `knock`**  | Knock accepted.      | Must never happen.                     | If the `state_key` is the same as the `sender`, the user retracted the knock. Otherwise, the `state_key` user had their knock denied.   | User was banned.            | No change.           |
 
 properties:
   content:
@@ -72,7 +72,7 @@ properties:
 
           Client and server implementations should be aware of the [signing implications](/rooms/v8/#authorization-rules) of including this
           field in further events: in particular, the event must be signed by the server which
-          owns the user ID in the field. When copying the membership event's `content` 
+          owns the user ID in the field. When copying the membership event's `content`
           (for profile updates and similar) it is therefore encouraged to exclude this
           field in the copy, as otherwise the event might fail event authorization.
       reason:

static/diagrams/membership.drawio

@@ -1 +1 @@
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