ovn-migration: Add doctext explaining VXLAN to Geneve migration

The Ansible command itself is hard to read, the patch adds some doctext
to improve understanding of the ansible task.

Change-Id: I4ab7a83da9bbf64ee6b19b2a0611fd64e09e2132
Signed-off-by: Jakub Libosvar <libosvar@redhat.com>

tools/ovn_migration/tripleo_environment/playbooks/roles/prepare-controllers/tasks/main.yml

@@ -4,6 +4,17 @@
   when: ovn_central is defined
   register: mysql_url
 
+# The shell below is not readable well. The code spawns a sqlalchemy engine
+# and connects to the Neutron database to run following SQL command:
+# UPDATE networksegments SET networksegments.network_type='geneve' WHERE networksegments.network_type='vxlan';
+# The indented Python code looks as follows:
+#
+# from sqlalchemy import create_engine
+#
+# engine = create_engine("{{ mysql_url.stdout }}")
+# with engine.connect() as conn:
+#     conn.execute("SQL COMMAND")
+#
 - name: Change vxlan networks to Geneve
   shell: podman exec -it neutron_api python3 -c $'from sqlalchemy import create_engine\nengine = create_engine("{{ mysql_url.stdout }}")\nwith engine.connect() as conn:\n\tconn.execute("update networksegments set networksegments.network_type=\'geneve\' where networksegments.network_type=\'vxlan\';")'
   when: ovn_central is defined