Add room version 11 (#1604)

* Remove duplicate words.

* Add information on room version 11.

* Note some event changes.

* Newsfragment

* Fix-up event schema.

* Apply suggestions from code review

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

* Fix 'new in this version'.

* Clarify creator field.

* Fix-up event format & examples.

* Move the Redactions section to the client section.

* Reference the sender instead of the creator.

* More links

* Even more links.

* Fix order of headers.

* Fix typos.

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

* Clarify description of creator.

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

* Clean-up intro paragraphs for redactions.

* Clean-up examples and language.

* Review comments.

* FIx-up markup tags.

---------

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

data/api/server-server/definitions/pdu.yaml

@@ -20,6 +20,10 @@ allOf:
   - $ref: "unsigned_pdu.yaml"
   - type: object
     properties:
+      redacts:
+        type: string
+        description: For redaction events, the ID of the event being redacted.
+        example: "$def456:matrix.org"
       hashes:
         type: object
         title: Event Hash