Better description of the mainline of a PL event.

changelogs/room_versions/newsfragments/1107.clarification

@@ -1 +1 @@
-More explicitly define the mainline of a power event and the mainline ordering of other events.
+For room versions 2 through 10: More explicitly define the mainline of a power event and the mainline ordering of other events.

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

@@ -88,8 +88,8 @@ Starting with *i* = 0, repeatedly fetch *P*<sub>*i*+1</sub>, the
 Increment *i* and repeat until *P<sub>i</sub>* has no `m.room.power_levels`
 event in its `auth_events`.
 The *mainline of P*<sub>0</sub> is the list of events
-    [*P*<sub>n</sub> , ... , *P*<sub>1</sub>, *P*<sub>0</sub>],
-ordered from oldest to newest.
+    [*P*<sub>0</sub> , *P*<sub>1</sub>, ... , *P<sub>n</sub>*],
+fetched in this way.
 
 Let *e* = *e<sub>0</sub>* be another event (possibly another
 `m.room.power_levels` event). We can compute a similar list of events
@@ -107,17 +107,18 @@ Now compare these two lists as follows.
 * If such a *j* exists, then *e<sub>j</sub>* = *P<sub>i</sub>* for some unique
   index *i* ≥ 0. Otherwise set *i* = ∞, where ∞ is a sentinel value greater
   than any integer.
-* In both cases, the *position of e in the mainline of P* is *i*.
+* In both cases, the *mainline position* of *e* is *i*.
 
 The *mainline ordering based on* *P* of a set of events is the ordering,
 from smallest to largest, using the following comparison relation on
 events: for events *x* and *y*, *x* &lt; *y* if
 
-1.  the position of *x* in the mainline of *P* is **greater** than
-    the position of *y* in the mainline of *P*; or
-2.  the positions of the events are the same, but *x*'s
+1.  the mainline position of *x* is **greater** than
+    the mainline position of *y* in the mainline of *P* (i.e. the auth chain of 
+*x* is based on an earlier event in the mainline than *y*); or
+2.  the mainline positions of the events are the same, but *x*'s
     `origin_server_ts` is *less* than *y*'s `origin_server_ts`; or
-3.  the positions of the events are the same and the events have the
+3.  the mainline positions of the events are the same and the events have the
     same `origin_server_ts`, but *x*'s `event_id` is *less* than *y*'s
     `event_id`.