Fix: Add notice about upgrading Mentat to documentation. (Redmine issue: #6800)

doc/sphinx/_doclib/upgrading.rst

@@ -227,6 +227,8 @@ from version ``11.x`` to ``12.x``.
     It can take a lot of time depending on the size of the current database,
     because the data files need to be converted to new format.
 
+    Upgrade to the latest version of Mentat prior to upgrading PostgreSQL.
+
 .. code-block:: shell
 
     # Launch tmux or screen.
@@ -343,6 +345,8 @@ from version ``12.x`` to ``13.x``.
     It can take a lot of time depending on the size of the current database,
     because the data files need to be converted to new format.
 
+    Upgrade to the latest version of Mentat prior to upgrading PostgreSQL.
+
 .. code-block:: shell
 
     # Launch tmux or screen.