docs: add more information when highest release is achieved. (canonic…

…al#448)

cou/steps/plan.py

@@ -183,8 +183,12 @@ def _verify_highest_release_achieved(analysis_result: Analysis) -> None:
     if o7k_release and current_series and str(o7k_release) == last_supported:
         raise HighestReleaseAchieved(
             f"No upgrades available for OpenStack {str(o7k_release).capitalize()} on "
-            f"Ubuntu {current_series.capitalize()}.\nNewer OpenStack releases "
-            "may be available after upgrading to a later Ubuntu series."
+            f"Ubuntu {current_series.capitalize()}.\nIn future releases of COU, newer "
+            "OpenStack releases may available after manually upgrading to a later Ubuntu series.\n"
+            "Charmed OpenStack Upgrader will not support upgrade across series,\n"
+            "please refer to the official documentation on how to do series upgrade:\n"
+            "- https://docs.openstack.org/charm-guide/latest/admin/upgrades/series.html\n"
+            "- https://docs.openstack.org/charm-guide/latest/admin/upgrades/series-openstack.html"
         )
 
 

tests/unit/steps/test_plan.py

@@ -541,8 +541,13 @@ def test_verify_highest_release_achieved():
     mock_analysis_result.current_cloud_o7k_release = OpenStackRelease("yoga")
     mock_analysis_result.current_cloud_series = "focal"
     exp_error_msg = (
-        "No upgrades available for OpenStack Yoga on Ubuntu Focal.\n"
-        "Newer OpenStack releases may be available after upgrading to a later Ubuntu series."
+        "No upgrades available for OpenStack Yoga on "
+        "Ubuntu Focal.\nIn future releases of COU, newer OpenStack releases "
+        "may available after manually upgrading to a later Ubuntu series.\n"
+        "Charmed OpenStack Upgrader will not support upgrade across series,\n"
+        "please refer to the official documentation on how to do series upgrade:\n"
+        "- https://docs.openstack.org/charm-guide/latest/admin/upgrades/series.html\n"
+        "- https://docs.openstack.org/charm-guide/latest/admin/upgrades/series-openstack.html"
     )
     with pytest.raises(HighestReleaseAchieved, match=exp_error_msg):
         cou_plan._verify_highest_release_achieved(mock_analysis_result)