Improved documentation based on the feedback from CTI project migrating their Mentat instance.

(Redmine issue: #3361)

doc/sphinx/_doclib/installation.rst

@@ -271,6 +271,14 @@ with `Shibboleth SSO <https://www.shibboleth.net/index/basic/>`__ login service
 Installation guide - Ansible roles
 --------------------------------------------------------------------------------
 
+.. note::
+
+    Please do not use Ansible roles for installation during migration from previous
+    stable version of Mentat based on Perl and MongoDB. Some of the configuration
+    files changed and you may encounter weird errors in case original configuration
+    stays active. Part of the migration process is assessing changes in configuration
+    files and this process is not automated and needs to be done by hand.
+
 If you are fond of `Ansible <https://www.ansible.com/>`__, there is great news
 for you: we have prepared `Ansible <https://www.ansible.com/>`__ roles to make
 the installation as simple as possible. These roles are available via official

doc/sphinx/_doclib/migration.rst

@@ -125,6 +125,15 @@ to do just that, so the whole migration process is as simple as executing them.
     # Step 0: Reconfigure Mentat system by comparing old and new configuration files.
     #         This should be done manually, because most of the modules have new
     #         configuration options and you should consider tweaking some of them.
+    #         This step is really important, you may encounter weird errors in case
+    #         some outdated configuration stays active, especially in following files:
+    #               - mentat-backup.py.conf
+    #               - mentat-cleanup.py.conf
+    #               - mentat-controller.py.conf
+    #               - mentat-dbmngr.py.conf
+    #               - mentat-netmngr.py.conf
+    #               - mentat-reporter.py.conf
+    #               - mentat-statistician.py.conf
 
     # Step 1: Migrate system metadata tables (users, groups, reports, statistics, etc.).
     /etc/mentat/scripts/sqldb-migrate-data.py