document that Config.from_object uses object as-is

docs/config.rst

@@ -553,6 +553,44 @@ To enable such a config you just have to call into
 
     app.config.from_object('configmodule.ProductionConfig')
 
+Note that :meth:`~flask.Config.from_object` does not instantiate the class
+object. If you need to instantiate the class, such as to access a property,
+then you must do so before calling :meth:`~flask.Config.from_object`::
+
+    from configmodule import ProductionConfig
+    app.config.from_object(ProductionConfig())
+
+    # Alternatively, import via string:
+    from werkzeug.utils import import_string
+    cfg = import_string('configmodule.ProductionConfig')()
+    app.config.from_object(cfg)
+
+Instantiating the configutation object allows you to use ``@property`` in
+your configuration classes::
+
+    class Config(object):
+        """Base config, uses staging database server."""
+        DEBUG = False
+        TESTING = False
+        DB_SERVER = '192.168.1.56'
+
+        @property
+        def DATABASE_URI(self):         # Note: all caps
+            return 'mysql://user@{}/foo'.format(self.DB_SERVER)
+
+    class ProductionConfig(Config):
+        """Uses production database server."""
+        DB_SERVER = '192.168.19.32'
+
+    class DevelopmentConfig(Config):
+        DB_SERVER = 'localhost'
+        DEBUG = True
+
+    class TestingConfig(Config):
+        DB_SERVER = 'localhost'
+        DEBUG = True
+        DATABASE_URI = 'sqlite:///:memory:'
+
 There are many different ways and it's up to you how you want to manage
 your configuration files.  However here a list of good recommendations: