Refactoring and cleanups

A few non-functional changes:

* A bunch of docstrings to document types
* Split `EventsStore._persist_events_txn` up a bit. Hopefully it's a bit more
  readable.
* Rephrase `EventFederationStore._update_min_depth_for_room_txn` to avoid
  mind-bending conditional.
* Rephrase rejected/outlier conditional in `_update_outliers_txn` to avoid
  mind-bending conditional.

synapse/events/snapshot.py

@@ -15,6 +15,32 @@
 
 
 class EventContext(object):
+    """
+    Attributes:
+        current_state_ids (dict[(str, str), str]):
+            The current state map including the current event.
+            (type, state_key) -> event_id
+
+        prev_state_ids (dict[(str, str), str]):
+            The current state map excluding the current event.
+            (type, state_key) -> event_id
+
+        state_group (int): state group id
+        rejected (bool|str): A rejection reason if the event was rejected, else
+            False
+
+        push_actions (list[(str, list[object])]): list of (user_id, actions)
+            tuples
+
+        prev_group (int): Previously persisted state group. ``None`` for an
+            outlier.
+        delta_ids (dict[(str, str), str]): Delta from ``prev_group``.
+            (type, state_key) -> event_id. ``None`` for an outlier.
+
+        prev_state_events (?): XXX: is this ever set to anything other than
+            the empty list?
+    """
+
     __slots__ = [
         "current_state_ids",
         "prev_state_ids",