Made Model.__eq__ consider proxy models equivalent

Fixed #11892, fixed #16458, fixed #14492.

docs/ref/models/instances.txt

@@ -494,6 +494,40 @@ using ``__str__()`` like this::
             # first_name and last_name will be unicode strings.
             return force_bytes('%s %s' % (self.first_name, self.last_name))
 
+``__eq__``
+----------
+
+.. method:: Model.__eq__()
+
+The equality method is defined such that instances with the same primary
+key value and the same concrete class are considered equal. The term
+concrete class means proxy model's first non-proxy parent or the class
+itself if it isn't a proxy class.
+
+For example::
+
+    form django.db import models
+
+    class MyModel(models.Model):
+        id = models.AutoField(primary_key=True)
+
+    class MyProxyModel(MyModel):
+        class Meta:
+            proxy = True
+
+    class MultitableInherited(MyModel):
+        pass
+
+    MyModel(id=1) == MyModel(id=1)
+    MyModel(id=1) == MyProxyModel(id=1)
+    MyModel(id=1) != MultitableInherited(id=1)
+    MyModel(id=1) != MyModel(id=2)
+
+.. versionchanged:: 1.7
+
+  In previous versions only instances of the exact same class and same
+  primary key value were considered equal.
+
 ``get_absolute_url``
 --------------------