Add mention of merged configs into documentation

(Redmine issue: #6949)

doc/sphinx/_doclib/upgrading.rst

@@ -43,6 +43,9 @@ to latest version:
     $ aptitude update
     $ aptitude upgrade
 
+    # Do not forget to review changes in configuration files/structure and
+    # adapt accordingly.
+
     # Step 3: To be safe activate the Python virtual environment for Mentat system:
     $ . /var/mentat/venv/bin/activate
 
@@ -98,6 +101,21 @@ to quickly turn maintenance mode ON/OFF. Upgrading steps are then much more simp
     $ /etc/mentat/scripts/maintenance-mode.sh off
 
 
+.. _section-upgrading-mentat-2-8:
+
+Upgrading to Mentat 2.8
+--------------------------------------------------------------------------------
+
+* Rulesets for two inspectors have been merged in to one instance. That
+  concerns mentat-inspector.py, mentat-inspector-b.py.conf and
+  mentat-controller.py.conf. If you use vanilla configuration from .deb,
+  package manager will replace the old configuration files with new ones
+  automatically. If you used different installation method or made some
+  modifications into these configuration files, you are encouraged to review
+  the changes and decide whether you want to stay with split configuration
+  or also proceed with the merge.
+
+
 .. _section-upgrading-postgresql-10:
 
 Upgrading PostgreSQL from 10.x to 11.x