Update fields.md explaining how the `source` argument affects `.validated_data` (#10008)

* Update fields.md explaining how the `source` argument affects structure of `.validated_data`

One may think that, after successful validation, a key in the `.validated_data` attribute of a serializer always equals the corresponding field name in the serializer. However, that may not be the case if the field has been defined in the serializer with a dotted path value in its `source` argument.  In that case, what is actually included in `.validated_data` is a nested dictionary that is derived by expanding the dotted path of the `source` argument. This addition in the documentation clarifies the situation and explains why DRF follows that behavior.

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Co-authored-by: Asif Saif Uddin {"Auvi":"অভি"} <auvipy@gmail.com>