Update docstrings

rest_framework/tests/fields.py

 """
-General tests for relational fields.
+General serializer field tests.
 """
 
 from django.db import models
@@ -29,15 +29,21 @@ class CharPrimaryKeyModelSerializer(serializers.ModelSerializer):
 class ReadOnlyFieldTests(TestCase):
     def test_auto_now_fields_read_only(self):
         """
-        auto_now and auto_now_add fields should be readonly by default.
+        auto_now and auto_now_add fields should be read_only by default.
         """
         serializer = TimestampedModelSerializer()
         self.assertEquals(serializer.fields['added'].read_only, True)
 
     def test_auto_pk_fields_read_only(self):
+        """
+        AutoField fields should be read_only by default.
+        """
         serializer = TimestampedModelSerializer()
         self.assertEquals(serializer.fields['id'].read_only, True)
 
     def test_non_auto_pk_fields_not_read_only(self):
+        """
+        PK fields other than AutoField fields should not be read_only by default.
+        """
         serializer = CharPrimaryKeyModelSerializer()
         self.assertEquals(serializer.fields['id'].read_only, False)