[IMP] developer/{howtos,reference}: update training to match v15 changes

- Reference v15 training solutions
- Refer to Command namespace instead of triplets (e.g.
  Command.create(values) instead of (0, 0, values)
- Add notice about OWL transition
- Add reference to @api.ondelete (instead of override unlink())
- Remove references to SavepointCase (now only TransactionCase) from
  unit test topic

- Also add missing reference documentation for ondelete

closes odoo/documentation#1146

Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

content/developer/reference/addons/orm.rst

@@ -611,7 +611,7 @@ Method decorators
 =================
 
 .. automodule:: odoo.api
-    :members: depends, depends_context, constrains, onchange, returns, autovacuum, model, model_create_multi
+    :members: depends, depends_context, constrains, onchange, returns, autovacuum, model, model_create_multi, ondelete
 
 .. .. currentmodule:: odoo.api