docs: explain missing preference header

docs/references/api/preferences.rst

@@ -12,7 +12,7 @@ The following preferences are supported.
 - ``Prefer: return``. See :ref:`prefer_return`.
 - ``Prefer: count``. See :ref:`prefer_count`.
 - ``Prefer: resolution``. See :ref:`prefer_resolution`.
-- ``Prefer: missing``. See :ref:`bulk_insert_default`.
+- ``Prefer: missing``. See :ref:`prefer_missing`.
 - ``Prefer: max-affected``, See :ref:`prefer_max_affected`.
 - ``Prefer: tx``. See :ref:`prefer_tx`.
 
@@ -196,6 +196,47 @@ The ``tx`` preference can be set to specify if the :ref:`transaction <transactio
   {"id": 35, "name": "Project X"}
 
 
+.. _prefer_missing:
+
+Missing
+=======
+
+When doing ``POST`` and ``PATCH`` requests, any missing columns in the payload will be inserted as ``null`` value by default. To use the ``DEFAULT`` column value instead, use the ``Prefer: missing=default`` header.
+
+Having:
+
+.. code-block:: postgres
+
+  create table foo (
+    id bigint generated by default as identity primary key
+  , bar text
+  , baz int default 100
+  );
+
+A request:
+
+.. code-block:: bash
+
+  curl "http://localhost:3000/foo?columns=id,bar,baz" \
+    -H "Content-Type: application/json" \
+    -H "Prefer: missing=default, return=representation" \
+    -d @- << EOF
+      [
+        { "bar": "val1" },
+        { "bar": "val2", "baz": 15 }
+      ]
+  EOF
+
+Will result in:
+
+.. code-block:: json
+
+  [
+    { "id":  1, "bar": "val1", "baz": 100 },
+    { "id":  2, "bar": "val2", "baz": 15 }
+  ]
+
+
 .. _prefer_max_affected:
 
 Max Affected