Fixed #20625 -- Chainable Manager/QuerySet methods.

Additionally this patch solves the orthogonal problem that specialized
`QuerySet` like `ValuesQuerySet` didn't inherit from the current `QuerySet`
type. This wasn't an issue until now because we didn't officially support
custom `QuerySet` but it became necessary with the introduction of this new
feature.

Thanks aaugustin, akaariai, carljm, charettes, mjtamlyn, shaib and timgraham
for the reviews.

docs/ref/models/querysets.txt

@@ -121,9 +121,7 @@ described here.
 QuerySet API
 ============
 
-Though you usually won't create one manually — you'll go through a
-:class:`~django.db.models.Manager` — here's the formal declaration of a
-``QuerySet``:
+Here's the formal declaration of a ``QuerySet``:
 
 .. class:: QuerySet([model=None, query=None, using=None])
 
@@ -1866,6 +1864,17 @@ DO_NOTHING do not prevent taking the fast-path in deletion.
 Note that the queries generated in object deletion is an implementation
 detail subject to change.
 
+as_manager
+~~~~~~~~~~
+
+.. classmethod:: as_manager()
+
+.. versionadded:: 1.7
+
+Class method that returns an instance of :class:`~django.db.models.Manager`
+with a copy of the ``QuerySet``'s methods. See
+:ref:`create-manager-with-queryset-methods` for more details.
+
 .. _field-lookups:
 
 Field lookups