update doc based on postgrest/issues/1253 (#206)

api.rst

@@ -970,6 +970,10 @@ You can make an UPSERT with :code:`POST` and the :code:`Prefer: resolution=merge
 
 UPSERT operates based on the primary key columns, you must specify all of them. You can also choose to ignore the duplicates with :code:`Prefer: resolution=ignore-duplicates`. UPSERT works best when the primary key is natural, but it's also possible to use it if the primary key is surrogate (example: "id serial primary key"). For more details read `this issue <https://github.com/PostgREST/postgrest/issues/1118>`_.
 
+.. important::
+  After creating a table or changing its primary key, you must refresh PostgREST schema cache for UPSERT to work properly. To learn how to refresh the cache see :ref:`schema_reloading`.
+   
+
 A single row UPSERT can be done by using :code:`PUT` and filtering the primary key columns with :code:`eq`:
 
 .. code-block:: http