edit_history: Add comments detailing when user_id can be null.

This is already in the API documentation, but I think it's worth
having here.

zerver/lib/types.py

@@ -96,6 +96,10 @@ class EditHistoryEvent(TypedDict, total=False):
     Database format for edit history events.
     """
 
+    # user_id is null for precisely those edit history events
+    # predating March 2017, when we started tracking the person who
+    # made edits, which is still years after the introduction of topic
+    # editing support in Zulip.
     user_id: Optional[int]
     timestamp: int
     prev_stream: int
@@ -112,6 +116,7 @@ class FormattedEditHistoryEvent(TypedDict, total=False):
     Extended format used in the edit history endpoint.
     """
 
+    # See EditHistoryEvent for details on when this can be null.
     user_id: Optional[int]
     timestamp: int
     prev_stream: int