Fixed #16855 -- select_related() chains as expected.

select_related('foo').select_related('bar') is now equivalent to
select_related('foo', 'bar').

Also reworded docs to recommend select_related(*fields) over select_related()

django/db/models/sql/query.py

@@ -1712,7 +1712,10 @@ class Query(object):
         certain related models (as opposed to all models, when
         self.select_related=True).
         """
-        field_dict = {}
+        if isinstance(self.select_related, bool):
+            field_dict = {}
+        else:
+            field_dict = self.select_related
         for field in fields:
             d = field_dict
             for part in field.split(LOOKUP_SEP):