Add base doc for DurationField format

Author: sevdog

docs/api-guide/fields.md

@@ -377,13 +377,16 @@ A Duration representation.
 Corresponds to `django.db.models.fields.DurationField`
 
 The `validated_data` for these fields will contain a `datetime.timedelta` instance.
-The representation is a string following this format `'[DD] [HH:[MM:]]ss[.uuuuuu]'`.
 
-**Signature:** `DurationField(max_value=None, min_value=None)`
+**Signature:** `DurationField(format=api_settings.DURATION_FORMAT, max_value=None, min_value=None)`
 
+* `format` - A string representing the output format.  If not specified, this defaults to the same value as the `DURATION_FORMAT` settings key, which will be `'standard'` unless set. Setting to a format string indicates that `to_representation` return values should be coerced to string output. Format strings are described below. Setting this value to `None` indicates that Python `timedelta` objects should be returned by `to_representation`. In this case the date encoding will be determined by the renderer.
 * `max_value` Validate that the duration provided is no greater than this value.
 * `min_value` Validate that the duration provided is no less than this value.
 
+#### `DurationField` format strings
+Format strings may either be the special string `'iso-8601'`, which indicates that [ISO 8601][iso8601] style intervals should be used (eg `'P4DT1H15M20S'`), or the special string `'standard'`, which indicates that Django interval format `'[DD] [HH:[MM:]]ss[.uuuuuu]'` hould be used (eg: `'4 1:15:20'`).
+
 ---
 
 # Choice selection fields
@@ -552,7 +555,7 @@ For further examples on `HiddenField` see the [validators](validators.md) docume
 
 ---
 
-**Note:** `HiddenField()` does not appear in `partial=True` serializer (when making `PATCH` request). This behavior might change in future, follow updates on [github discussion](https://github.com/encode/django-rest-framework/discussions/8259). 
+**Note:** `HiddenField()` does not appear in `partial=True` serializer (when making `PATCH` request). This behavior might change in future, follow updates on [github discussion](https://github.com/encode/django-rest-framework/discussions/8259).
 
 ---
 

docs/api-guide/settings.md

@@ -314,6 +314,15 @@ May be a list including the string `'iso-8601'` or Python [strftime format][strf
 
 Default: `['iso-8601']`
 
+
+#### DURATION_FORMAT
+
+A format string that should be used by default for rendering the output of `DurationField` serializer fields.  If `None`, then `DurationField` serializer fields will return Python `timedelta` objects, and the duration encoding will be determined by the renderer.
+
+May be any of `None`, `'iso-8601'` or `'standard'` (the format accepted by `django.utils.dateparse.parse_duration`).
+
+Default: `'standard'`
+
 ---
 
 ## Encodings