[1.5.x] Fixed #19639 - Updated contributing to reflect model choices best practices.

Thanks charettes.

Backport of eaa716a413 from master

docs/internals/contributing/writing-code/coding-style.txt

@@ -136,14 +136,17 @@ Model style
   * ``def get_absolute_url()``
   * Any custom methods
 
-* If ``choices`` is defined for a given model field, define the choices as
-  a tuple of tuples, with an all-uppercase name, either near the top of
-  the model module or just above the model class. Example::
+* If ``choices`` is defined for a given model field, define each choice as
+  a tuple of tuples, with an all-uppercase name as a class attribute on the
+  model. Example::
 
-      DIRECTION_CHOICES = (
-          ('U', 'Up'),
-          ('D', 'Down'),
-      )
+    class MyModel(models.Model):
+        DIRECTION_UP = 'U'
+        DIRECTION_DOWN = 'D'
+        DIRECTION_CHOICES = (
+            (DIRECTION_UP, 'Up'),
+            (DIRECTION_DOWN, 'Down'),
+        )
 
 Use of ``django.conf.settings``
 -------------------------------