Clarify 'to_internal_value()' validation behavior (#5466)

Author: Ryan P Kilby

docs/api-guide/serializers.md

@@ -993,7 +993,7 @@ The following class is an example of a generic serializer that can handle coerci
 
 ## Overriding serialization and deserialization behavior
 
-If you need to alter the serialization, deserialization or validation of a serializer class you can do so by overriding the `.to_representation()` or `.to_internal_value()` methods.
+If you need to alter the serialization or deserialization behavior of a serializer class, you can do so by overriding the `.to_representation()` or `.to_internal_value()` methods.
 
 Some reasons this might be useful include...
 
@@ -1011,7 +1011,7 @@ Takes the object instance that requires serialization, and should return a primi
 
 Takes the unvalidated incoming data as input and should return the validated data that will be made available as `serializer.validated_data`. The return value will also be passed to the `.create()` or `.update()` methods if `.save()` is called on the serializer class.
 
-If any of the validation fails, then the method should raise a `serializers.ValidationError(errors)`. Typically the `errors` argument here will be a dictionary mapping field names to error messages.
+If any of the validation fails, then the method should raise a `serializers.ValidationError(errors)`. The `errors` argument should be a dictionary mapping field names (or `settings.NON_FIELD_ERRORS_KEY`) to a list of error messages. If you don't need to alter deserialization behavior and instead want to provide object-level validation, it's recommended that you intead override the [`.validate()`](#object-level-validation) method.
 
 The `data` argument passed to this method will normally be the value of `request.data`, so the datatype it provides will depend on the parser classes you have configured for your API.
 
@@ -1155,7 +1155,7 @@ The [html-json-forms][html-json-forms] package provides an algorithm and seriali
 
 ## QueryFields
 
-[djangorestframework-queryfields][djangorestframework-queryfields] allows API clients to specify which fields will be sent in the response via inclusion/exclusion query parameters.  
+[djangorestframework-queryfields][djangorestframework-queryfields] allows API clients to specify which fields will be sent in the response via inclusion/exclusion query parameters.
 
 ## DRF Writable Nested