Document changes to schema version 61 - 64 (#10917)

As pointed out by @richvdh, #10838 (comment)

Retroactively summarize `61` - `64`

changelog.d/10917.misc

@@ -0,0 +1 @@
+Document and summarize changes in schema version `61` - `64`.

synapse/storage/schema/__init__.py

@@ -27,11 +27,22 @@
 Changes in SCHEMA_VERSION = 61:
     - The `user_stats_historical` and `room_stats_historical` tables are not written and
       are not read (previously, they were written but not read).
+    - MSC2716: Add `insertion_events` and `insertion_event_edges` tables to keep track
+      of insertion events in order to navigate historical chunks of messages.
+    - MSC2716: Add `chunk_events` table to track how the chunk is labeled and
+      determines which insertion event it points to.
+
+Changes in SCHEMA_VERSION = 62:
+    - MSC2716: Add `insertion_event_extremities` table that keeps track of which
+      insertion events need to be backfilled.
 
 Changes in SCHEMA_VERSION = 63:
     - The `public_room_list_stream` table is not written nor read to
       (previously, it was written and read to, but not for any significant purpose).
       https://github.com/matrix-org/synapse/pull/10565
+
+Changes in SCHEMA_VERSION = 64:
+    - MSC2716: Rename related tables and columns from "chunks" to "batches".
 """