Clarify documentation and error message about keep_untouched (#926)

* Clarify keep_untouched documentation (#924)

* Clarify error message for custom types (#924)

* Fix tests for changed error message (#924)

* fix formatting

* remove erroneous error message and add change

changes/926-retnikt.md

@@ -0,0 +1 @@
+Clarify usage of `remove_untouched`, improve error message for types with no validators.

docs/usage/model_config.md

@@ -62,8 +62,8 @@ Options:
 : a callable that takes a field name and returns an alias for it
 
 **`keep_untouched`**
-: a tuple of types (e.g. descriptors) that should not be changed during model creation and will not be
-  included in the model schemas
+: a tuple of types (e.g. descriptors) for a model's default values that should not be changed during model creation and will
+not be included in the model schemas. **Note**: this means that attributes on the model with *defaults of this type*, not *annotations of this type*, will be left alone.
   
 **`schema_extra`**
 : a `dict` used to extend/update the generated JSON Schema

pydantic/validators.py

@@ -568,9 +568,7 @@ def find_validators(  # noqa: C901 (ignore complexity)
     if config.arbitrary_types_allowed:
         yield make_arbitrary_type_validator(type_)
     else:
-        raise RuntimeError(
-            f'no validator found for {type_} see `keep_untouched` or `arbitrary_types_allowed` in Config'
-        )
+        raise RuntimeError(f'no validator found for {type_}, see `arbitrary_types_allowed` in Config')
 
 
 def _find_supertype(type_: AnyType) -> Optional[AnyType]:

tests/test_main.py

@@ -953,7 +953,7 @@ def test_custom_types_fail_without_keep_untouched():
 
     assert str(e.value) == (
         "no validator found for <class 'tests.test_main.test_custom_types_fail_without_keep_untouched.<locals>."
-        "_ClassPropertyDescriptor'> see `keep_untouched` or `arbitrary_types_allowed` in Config"
+        "_ClassPropertyDescriptor'>, see `arbitrary_types_allowed` in Config"
     )
 
     class Model(BaseModel):