Update important-upgrade-notes.rst (#4811)

* Update important-upgrade-notes.rst

* Update important-upgrade-notes.rst

* Update important-upgrade-notes.rst

* Fix typos

Author: amyblais

source/upgrade/important-upgrade-notes.rst

@@ -3,12 +3,22 @@ Important Upgrade Notes
 
 .. important::
    - Support for Mattermost Server v5.31 `Extended Support Release <https://docs.mattermost.com/administration/extended-support-release.html>`_ will come to the end of its life cycle on October 15, 2021. Upgrading to Mattermost Server v5.37 `Extended Support Release <https://docs.mattermost.com/administration/extended-support-release.html>`_ or later is required.
-   - In the v5.38 release (August 16, 2021), we will deprecate "config watcher" (the mechanism that automatically reloads the ``config.json file``), in favor of an mmctl command that will need to be run to apply configuration changes after they are made. This change will improve configuration performance and robustness.
    - The deprecations `listed here <https://docs.mattermost.com/administration/changelog.html#upcoming-deprecations-in-mattermost-v6-0>`_ are planned for the Mattermost v6.0 release, which is scheduled for September 15, 2021. This list is subject to change prior to the release.
+   - Upgrading Mattermost can result in an error “ERROR: default for column "column_name" cannot be cast automatically to type jsonb” if you have non-JSON data in a column. This can also happen if the database data has been manipulated by external processes (e.g. plugins) and in cases where improper input data could get silently truncated due to varchar limits. The data in the affected database column has to be manually fixed.
 
 +----------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | If you’re upgrading from a version earlier than... | Then...                                                                                                                                                          |
 +====================================================+==================================================================================================================================================================+
+| v5.38.0                                            | The “config watcher” (the mechanism that automatically reloads the ``config.json`` file) has been removed in favor of the ``mmctl config`` command that will     |
+|                                                    | need to be run to apply configuration changes after they are made. This change improves configuration performance and robustness.                                |
+|                                                    +------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+|                                                    | v5.38 adds fixes for some of the incorrect mention counts and unreads around threads and channels since the introduction of Collapsed Reply Threads (Beta). This |
+|                                                    | fix is done through a SQL migration, and it may take several minutes to complete for large databases. The ``fixCRTChannelMembershipCounts`` fix takes 1 minute   |
+|                                                    | and 20 seconds for a database containing approximately 4 million channel memberships and about 130,000 channels. The ``fixCRTThreadCountsAndUnreads`` fix takes  |
+|                                                    | about 3 minutes and 30 seconds for a database containing 56367 threads, 124587 thread memberships, and 220801 channel memberships. These are on MySQL v5.6.51.   | 
+|                                                    +------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+|                                                    | Focalboard v0.8.0 (released with Mattermost v5.38.0) requires Mattermost v5.37 due to the new database connection system.                                        |
++----------------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | v5.37.0                                            | The ``platform`` binary and “--platform” flag have been removed. If you are using the “--platform” flag or are using the ``platform`` binary directly to run     |
 |                                                    | the Mattermost server application via a systemd file or custom script, you will be required to use only the mattermost binary.                                   |
 |                                                    +------------------------------------------------------------------------------------------------------------------------------------------------------------------+