Fix unintentional stateres change added in #1042 (#1158)

* Fix unintentional stateres change added in #1042

* Changelog

changelogs/room_versions/newsfragments/1158.clarification

@@ -0,0 +1 @@
+For room versions 2–10: correct a mistaken clarification to the state resolution algorithm.

content/rooms/fragments/v2-state-res.md

@@ -138,9 +138,11 @@ the auth event is not rejected.
 
 The *resolution* of a set of states is obtained as follows:
 
-1.  Select all *power events* that appear in the *full conflicted set*. Compute
-    the union of their auth chains, including the power events themselves.
-    Sort the union using the *reverse topological power ordering*.
+1.  Select the set *X* of all *power events* that appear in the *full
+    conflicted set*. For each such power event *P*, enlarge *X* by adding
+    the events in the auth chain of *P* which also belong to the full
+    conflicted set. Sort $X$ into a list using the *reverse topological
+    power ordering*.
 2.  Apply the *iterative auth checks algorithm*, starting from the
     *unconflicted state map*, to the list of events from the previous
     step to get a partially resolved state.