Unify & standardize the v1.3 changelog (#1102)

* Reference MSCs where MSCs were not being referenced.
* Alter language to appear consistent and from a single voice.
* Bundle and group various changes together (will affect the final changelog - the rendered one still doesn't bundle appropriately).
* Move entries to the spec area they are intended to be in.

changelogs/room_versions/newsfragments/1037.clarification

@@ -1,2 +1 @@
-Improve readability of definitions in the state resolution v2 algorithm.
-
+Improve readability and understanding of the state resolution algorithms.

changelogs/room_versions/newsfragments/1042.clarification

@@ -0,0 +1 @@
+Improve readability and understanding of the state resolution algorithms.

changelogs/room_versions/newsfragments/1043.clarification

@@ -1 +1 @@
-Adjust mathematical notation used in the description of state resolution to render better in browsers.
+Improve readability and understanding of the state resolution algorithms.

changelogs/room_versions/newsfragments/1050.clarification

@@ -1 +1 @@
-Add cross-references to PDU content definitions from the authorisation rules.
+Improve readability of the authorization rules.

changelogs/room_versions/newsfragments/1093.clarification

@@ -1 +1 @@
-Auth rules: clarify that the resident server must sign a restricted join event.
+For room versions 8, 9, and 10: clarify which homeserver is required to sign the join event.

changelogs/room_versions/newsfragments/3737.clarification

@@ -1 +1 @@
-Fix join membership auth rules when `join_rule` is `knock`.
+For room versions 7, 8, 9, and 10: fix join membership authorization rules when `join_rule` is `knock`.

changelogs/room_versions/newsfragments/3739.feature

@@ -1 +1 @@
-Update the default room version to 9.
+Update the default room version to 9 as per [MSC3589](https://github.com/matrix-org/matrix-spec-proposals/pull/3589).