Clarify user model/register form interaction in docs.

docs/customizing.rst

@@ -75,6 +75,18 @@ register form or override validators::
     security = Security(app, user_datastore,
              register_form=ExtendedRegisterForm)
 
+For the ``register_form`` and ``confirm_register_form``, each field is
+passed to the user model when a user is created. In the above case,
+the ``first_name`` and ``last_name`` fields are passed directly to the
+model, so the model should look like::
+
+    class User(db.Model, UserMixin):
+        id = db.Column(db.Integer, primary_key=True)
+        email = db.Column(db.String(255), unique=True)
+        password = db.Column(db.String(255))
+        first_name = db.Column(db.String(255))
+        last_name = db.Column(db.String(255))
+
 The following is a list of all the available form overrides:
 
 * ``login_form``: Login form