                  # If we can't find a field on the model that matches, it could
                  # be an extra field on the form.
                  return []
              else:
                  if (isinstance(field, models.ManyToManyField) and
                          not field.remote_field.through._meta.auto_created):
                      return [
                          checks.Error(
                              "The value of '%s' cannot include the ManyToManyField '%s', "
                              "because that field manually specifies a relationship model."
                              % (label, field_name),
 
</s> remove                             "The value of '%s' cannot include the ManyToManyField '%s', "
 </s> add                             "The value of '%s' cannot include the many-to-many field '%s' " </s> remove                 "The value of 'fields' cannot include the ManyToManyField 'authors', "
 </s> add                 "The value of 'fields' cannot include the many-to-many field 'authors' " </s> remove                 "The value of 'fieldsets[1][1][\"fields\"]' cannot include the ManyToManyField "
                "'authors', because that field manually specifies a relationship model.",
 </s> add                 "The value of 'fieldsets[1][1][\"fields\"]' cannot include the many-to-many field "
                "'authors' because that field manually specifies a relationship model.", </s> remove                         "ForeignKey or ManyToManyField." % (label, field_name),
 </s> add                         "a foreign key, or a many-to-many field." % (label, field_name), </s> remove   ``ManyToManyField`` ``<field name>``, because that field manually specifies a
 </s> add   many-to-many field ``<field name>``, because that field manually specifies a
                if field.many_to_many and not field.remote_field.through._meta.auto_created:
                  # If we can't find a field on the model that matches, it could
                  # be an extra field on the form.
                  return []
              else:
                if field.many_to_many and not field.remote_field.through._meta.auto_created:
                if field.many_to_many and not field.remote_field.through._meta.auto_created:
                      return [
                          checks.Error(
                              "The value of '%s' cannot include the ManyToManyField '%s', "
                              "because that field manually specifies a relationship model."
                              % (label, field_name),
keep keep keep keep replace replace keep keep keep keep keep
 Refs #24227 -- Replaced M2M isinstance checks by field.many_to_many

Thanks Markus Holtermann, Collin Anderson and Tim Graham for the reviews. 
django/contrib/admin/checks.py