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/fragments/v1-redactions.md

@@ -0,0 +1,29 @@
+Upon receipt of a redaction event, the server must strip off any keys
+not in the following list:
+
+-   `event_id`
+-   `type`
+-   `room_id`
+-   `sender`
+-   `state_key`
+-   `content`
+-   `hashes`
+-   `signatures`
+-   `depth`
+-   `prev_events`
+-   `prev_state`
+-   `auth_events`
+-   `origin`
+-   `origin_server_ts`
+-   `membership`
+
+The content object must also be stripped of all keys, unless it is one
+of one of the following event types:
+
+-   `m.room.member` allows key `membership`.
+-   `m.room.create` allows key `creator`.
+-   `m.room.join_rules` allows key `join_rule`.
+-   `m.room.power_levels` allows keys `ban`, `events`, `events_default`,
+    `kick`, `redact`, `state_default`, `users`, `users_default`.
+-   `m.room.aliases` allows key `aliases`.
+-   `m.room.history_visibility` allows key `history_visibility`.