Clarify validate_default and _Unset handling in usage docs and migrat…

…ion guide (#6950)

docs/migration.md

@@ -238,6 +238,10 @@ See [Model Config](usage/model_config.md) for more details.
         for the annotated type will _also_ be applied even to defaults, not just the custom validators. For
         example, despite the fact that the validator below will never error, the following code raises a `ValidationError`:
 
+!!! note
+    To avoid this, you can use the `validate_default` argument in the `Field` function. When set to `True`, it mimics the behavior of `always=True` in Pydantic v1. However, the new way of using `validate_default` is encouraged as it provides more flexibility and control.
+
+
 ```python test="skip"
 from pydantic import BaseModel, validator
 

docs/usage/validators.md

@@ -266,7 +266,8 @@ print(context['logs'])
 
 Validators won't run when the default value is used.
 This applies both to `@field_validator` validators and `Annotated` validators.
-You can force them to run with `Field(validate_defaults=True)`, but you are generally better off using a `@model_validator(mode='before')`.
+You can force them to run with `Field(validate_defaults=True)`. Setting `validate_default` to `True` has the closest behavior to using `always=True` in `validator` in Pydantic v1. However, you are generally better off using a `@model_validator(mode='before')` where the function is called before the inner validator is called.
+
 
 ```py
 from typing_extensions import Annotated

pydantic/fields.py

@@ -701,6 +701,9 @@ def Field(  # noqa: C901
     Used to provide extra information about a field, either for the model schema or complex validation. Some arguments
     apply only to number fields (`int`, `float`, `Decimal`) and some apply only to `str`.
 
+    Note:
+        - Any `_Unset` objects will be replaced by the corresponding value defined in the `_DefaultValues` dictionary. If a key for the `_Unset` object is not found in the `_DefaultValues` dictionary, it will default to `None`
+
     Args:
         default: Default value if the field is not set.
         default_factory: A callable to generate the default value, such as :func:`~datetime.utcnow`.
@@ -717,7 +720,7 @@ def Field(  # noqa: C901
         discriminator: Field name for discriminating the type in a tagged union.
         json_schema_extra: Any additional JSON schema data for the schema property.
         frozen: Whether the field is frozen.
-        validate_default: Run validation that isn't only checking existence of defaults. `True` by default.
+        validate_default: Run validation that isn't only checking existence of defaults. This can be set to `True` or `False`. If not set, it defaults to `None`.
         repr: A boolean indicating whether to include the field in the `__repr__` output.
         init_var: Whether the field should be included in the constructor of the dataclass.
         kw_only: Whether the field should be a keyword-only argument in the constructor of the dataclass.