Add some comments

synapse/storage/databases/main/end_to_end_keys.py

@@ -1317,6 +1317,10 @@ def _claim_e2e_one_time_key_simple(
             OTK was found.
         """
 
+        # Return the oldest keys from this device (based on `ts_added_ms`).
+        # Doing so means that keys are issued in the same order they were uploaded,
+        # which reduces the chances of a client expiring its copy of a (private)
+        # key while the public key is still on the server, waiting to be issued.
         sql = """
             SELECT key_id, key_json FROM e2e_one_time_keys_json
             WHERE user_id = ? AND device_id = ? AND algorithm = ?
@@ -1362,6 +1366,12 @@ def _claim_e2e_one_time_keys_bulk(
             A list of tuples (user_id, device_id, algorithm, key_id, key_json)
             for each OTK claimed.
         """
+        # Find, delete, and return the oldest keys from each device (based on
+        # `ts_added_ms`).
+        #
+        # Doing so means that keys are issued in the same order they were uploaded,
+        # which reduces the chances of a client expiring its copy of a (private)
+        # key while the public key is still on the server, waiting to be issued.
         sql = """
             WITH claims(user_id, device_id, algorithm, claim_count) AS (
                 VALUES ?