docs(pydocs): full python wrapper documentation

docs/fields.rst

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+Field selection
+===============
+
+The Szurubooru API supports *Field Selection*. This allows you to select only a subset of fields to return from the API. By default all fields are selected and returned, but if you
+pass in a list of field names to any of the client methods that support it, only those fields will be populated in the models.
+
+Example
+-------
+
+```python
+tags = client.list_tags(fields=["version", "names", "category"])
+assert tags.results[0].description is None
+```
+
+Because the ``description`` field was not specified, it's set to ``None``