Add upgrade considerations for the changed audit log timestamps

See wagtail#9590

docs/releases/5.1.md

@@ -364,3 +364,17 @@ Additionally, two new events will be dispatched when the dialog visibility chang
 | ------ | ----------------- |
 | Show   | `w-dialog:shown`  |
 | Hide   | `w-dialog:hidden` |
+
+### JSON-timestamps stored in `ModelLogEntry` and `PageLogEntry` are now ISO-formatted and UTC
+
+Previously, timestamps stored in the "data"-`JSONField` of `ModelLogEntry` and `PageLogEntry` have used the custom python format `%d %b %Y %H:%M`. Additionally, the `"go_live_at"` timestamp had been stored with the configured local timezone, instead of UTC.
+
+This has now been fixed, all timestamps are now stored as UTC, and because the "data"-`JSONField` now uses Django's `DjangoJSONEncoder`, those `datetime` objects are now automatically converted to the ISO format. This release contains a new migration `0088_fix_log_entry_json_timestamps` which converts all existing timestamps used by Wagtail to the new format.
+
+If you've developed your own subclasses of `ModelLogEntry`, `PageLogEntry` or `BaseLogEntry`, or used those existing models to create custom log entries, and you've stored timestamps similarly to Wagtail's old implementation (using `strftime("%d %b %Y %H:%M")`). You may want to adapt the storage of those timestamps to a consistent format too.
+
+There are probably three places in your code, which have to be changed:
+
+1. Creation: Instead of using `strftime("%d %b %Y %H:%M")`, you can now store the datetime directly in the "data" field. We've implemented a new helper `wagtail.utils.timestamps.ensure_utc()`, which ensures the correct timezone (UTC).
+2. Display: To display the timestamp in the user's timezone and format with a `LogFormatter`, we've created utils to parse (`wagtail.utils.timestamps.parse_datetime_localized()`) and render (`wagtail.utils.timestamps.render_timestamp()`) those timestamps. Look at the existing formatters [here](https://github.com/wagtail/wagtail/blob/main/wagtail/wagtail_hooks.py).
+3. Migration: You can use the code of the above migration ([source](https://github.com/wagtail/wagtail/blob/main/wagtail/migrations/0088_fix_log_entry_json_timestamps.py)) as a guideline to migrate your existing timestamps in the database.