Add upgrade notes entry on optional field data migration utility script

docs/source/upgrade_notes.rst

@@ -3,10 +3,69 @@
 Upgrade Notes
 =============
 
+|st2| v3.5
+----------
+
+.. _ref-upgrade-notes-v3-5:
+
+* Underlying database field type for storing large values such as action execution result has
+  changed for various database models (ActionExecutionDB, LiveActionDB, WorkflowExecutionDB,
+  TaskExecutionDB, TriggerInstanceDB).
+
+  For most users this change will result in 8-20x speed up when working with (reading and writing)
+  large values from / to the database.
+
+  The change is fully transparent to the end user and new objects created after upgrade to |st2|
+  v3.5 will automatically utilize this new field type.
+
+  Existing objects in the database will continue to utilize old field type.
+
+  If you want to migrate them to the new field type, you can use
+  ``st2-migrate-db-dict-field-values`` migration script which ships with |st2| v3.5. The script
+  only operates on "finalized" objects (i.e. finished executions) and it's idempotent which means
+  you can re-run it on failures or similar.
+
+  It's worth noting that running this script is optional - in most cases users primary care about
+  performance for recent / new objects (e.g. viewing recent executions) so if you don't migrate
+  existing database field values this means retrieving those objects will still be slow, but it
+  doesn't affect newly created objects post v3.5 upgrade which will utilize new field type and
+  as such, exhibit much better performance.
+
+  By default script will run in an interactive mode and display a prompt with warning which needs
+  to be acknowledged before continuing. If you want to run script in an non-interactive mode, pass
+  ``--yes`` command line argument to it.
+
+  Script also defaults to migrating data for the past 30 days. You can control time period for
+  objects which will be migrated using ``--start-dt`` and ``--end-dt`` argument as shown below.
+
+  The script currently doesn't support batching so in case you have many objects in the database
+  (especially trigger instances) you may need to migrate things in smaller chunks and call this
+  script multiple time (e.g. using a day long intervals or shorter).
+
+  Before running this script, you may also want to purge some old operational data. For information
+  on that, please refer to :doc:`Purging Old Operational Data </troubleshooting/purging_old_data>`
+  documentation page.
+
+  .. code-block:: bash
+
+    # Migrate objects with creation date between April 20th, 2021 and April 25th, 2021
+    /opt/stackstorm/st2/bin/st2-migrate-db-dict-field-values --start-dt "2021-04-20T19:16:55Z" --end-dt "2021-04-25T19:26:55Z"
+
+    # Migrate object between April 20th and "now"
+    /opt/stackstorm/st2/bin/st2-migrate-db-dict-field-values --start-dt "2021-04-20T19:16:55Z" --end-dt "now"
+
+  .. note::
+
+    You are strongly recommended to create full database backup before running this script.
+
+    If you run this migration script and a need arises, you won't be able to rollback back to a
+    previous version (v3.4) because code in previous version doesn't include support for this new
+    field type (in such case you would need to restore the database backup).
+
 .. _ref-upgrade-notes-v3-4:
 
 |st2| v3.4
--------------
+----------
 
 * Python 2 support was removed.
   Any packs that only support python 2 will need to be migrated to python 3.