Add docstring for `CustomValidatorMessage`

rest_framework/compat.py

@@ -299,6 +299,12 @@ except ImportError:
     DecimalValidator = None
 
 class CustomValidatorMessage(object):
+    """
+    We need to avoid evaluation of `lazy` translated `message` in `django.core.validators.BaseValidator.__init__`.
+    https://github.com/django/django/blob/75ed5900321d170debef4ac452b8b3cf8a1c2384/django/core/validators.py#L297
+    
+    Ref: https://github.com/encode/django-rest-framework/pull/5452
+    """
     def __init__(self, *args, **kwargs):
         self.message = kwargs.pop('message', self.message)
         super(CustomValidatorMessage, self).__init__(*args, **kwargs)