correct copy_on_model_validation docs (#4420)

* correct "copy_on_model_validation" docs, fix #4369

* Update docs/usage/model_config.md

* correct docs

changes/4369-samuelcolvin.md

@@ -0,0 +1 @@
+Update documentation for `copy_on_model_validation` to reflect recent changes

docs/usage/model_config.md

@@ -112,7 +112,12 @@ not be included in the model schemas. **Note**: this means that attributes on th
 : whether to treat any underscore non-class var attrs as private, or leave them as is; see [Private model attributes](models.md#private-model-attributes)
 
 **`copy_on_model_validation`**
-: whether inherited models used as fields should be reconstructed (copied) on validation instead of being kept untouched (default: `True`)
+: string literal to control how models instances are processed during validation, 
+with the following means (see [#4093](https://github.com/pydantic/pydantic/pull/4093) for a full discussion of the changes to this field):
+
+* `'none'` - models are not copied on validation, they're simply kept "untouched"
+* `'shallow'` - models are shallow copied, this is the default
+* `'deep'` - models are deep copied 
 
 **`smart_union`**
 : whether _pydantic_ should try to check all types inside `Union` to prevent undesired coercion; see [the dedicated section](#smart-union)