Fully specify room versions (#3432)

* Fully specify room versions

* Misc typo clarifications

* Try to clarify redactions a bit

* Update content/client-server-api/_index.md

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Update content/rooms/v6.md

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Update content/rooms/v6.md

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Better describe client considerations

* Doc template params

* Move redaction "new stuff" to v3

* Remove unhelpful sentences about "here follows unchanged stuff"

* Simplify event signing text

* Clean up handling redactions sections

* Move v4's event schema to unchanged section

* Update content/rooms/v4.md

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

content/rooms/v3.md

@@ -20,6 +20,10 @@ Clients should expect to see event IDs changed from the format of
 `$acR1l0raoZnm60CBwAVgqbZqoO/mYU81xysh1u7XcJk` (note the lack of domain
 and the potentially problematic slash).
 
+Though unchanged in this room version, clients which implement the
+redaction algorithm locally should refer to the [redactions](#redactions)
+section below for a full overview.
+
 ## Server implementation components
 
 {{% boxes/warning %}}
@@ -30,8 +34,8 @@ regarding client considerations is the resource that Client-Server API
 use cases should reference.
 {{% /boxes/warning %}}
 
-Room version 3 uses the state resolution algorithm defined in [room
-version 2](/rooms/v2), and the event format defined here.
+Room version 3 uses the event format described here in addition to
+all the remaining behaviour described by [room version 2](/rooms/v2).
 
 ### Event IDs
 
@@ -77,24 +81,34 @@ room version during a request.
 
 ### Authorization rules for events
 
-The authorization rules for a given event have changed in this room
-version due to the change in event format:
+{{% added-in this=true %}} `m.room.redaction` events are no longer
+explicitly part of the auth rules. They are still subject to the
+minimum power level rules, but should always fall into "11. Otherwise,
+allow". Instead of being authorized at the time of receipt, they are
+authorized at a later stage: see the [Handling Redactions](#handling-redactions)
+section below for more information.
 
--   The event no longer needs to be signed by the domain of the event ID
-    (as there is no domain in the event ID), but still needs to be
-    signed by the sender's domain.
--   In past room versions, redactions were only permitted to enter the
-    DAG if the sender's domain matched the domain in the event ID being
-    redacted, or the sender had appropriate permissions per the power
-    levels. Due to servers now not being able to determine where an
-    event came from during event authorization, redaction events are
-    always accepted (provided the event is allowed by `events` and
-    `events_default` in the power levels). However, servers should not
-    apply or send redactions to clients until both the redaction event
-    and original event have been seen, and are valid. Servers should
-    only apply redactions to events where the sender's domains match, or
-    the sender of the redaction has the appropriate permissions per the
-    power levels.
+<!-- set withVersioning=true so we get all the "new in this version" stuff -->
+{{% rver-fragment name="v3-auth-rules" withVersioning=true %}}
 
-The remaining rules are the same as [room version
-1](/rooms/v1#authorization-rules).
+### Handling redactions
+
+<!-- set withVersioning=true so we get all the "new in this version" stuff -->
+{{% rver-fragment name="v3-handling-redactions" withVersioning=true %}}
+
+## Unchanged from v2
+
+The following sections have not been modified since v2, but are included for
+completeness.
+
+### State resolution
+
+{{% rver-fragment name="v2-state-res" %}}
+
+### Canonical JSON
+
+{{% rver-fragment name="v1-canonical-json" %}}
+
+### Redactions
+
+{{% rver-fragment name="v1-redactions" %}}