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/v4-event-explainer.md

@@ -0,0 +1,15 @@
+The event ID is the [reference
+hash](/server-server-api#calculating-the-reference-hash-for-an-event) of
+the event encoded using a variation of [Unpadded
+Base64](/appendices#unpadded-base64) which replaces the 62nd and
+63rd characters with `-` and `_` instead of using `+` and `/`. This
+matches [RFC4648's definition of URL-safe
+base64](https://tools.ietf.org/html/rfc4648#section-5). Event IDs are
+still prefixed with `$` and may result in looking like
+`$Rqnc-F-dvnEYJTyHq_iKxU2bZ1CI92-kuZq3a5lr5Zg`.
+
+Just like in room version 3, event IDs should not be sent over
+federation to servers when the room uses this room version. On the
+receiving end of an event, the server should compute the relevant event
+ID for itself. Room version 3 also changes the format of `auth_events`
+and `prev_events` in a PDU.