Add base doc for DurationField format

Author: sevdog

docs/api-guide/fields.md

@@ -78,7 +78,7 @@ Defaults to `False`
 
 ### `source`
 
-The name of the attribute that will be used to populate the field.  May be a method that only takes a `self` argument, such as `URLField(source='get_absolute_url')`, or may use dotted notation to traverse attributes, such as `EmailField(source='user.email')`. 
+The name of the attribute that will be used to populate the field.  May be a method that only takes a `self` argument, such as `URLField(source='get_absolute_url')`, or may use dotted notation to traverse attributes, such as `EmailField(source='user.email')`.
 
 When serializing fields with dotted notation, it may be necessary to provide a `default` value if any object is not present or is empty during attribute traversal. Beware of possible n+1 problems when using source attribute if you are accessing a relational orm model. For example:
 
@@ -388,13 +388,16 @@ 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

docs/api-guide/settings.md

@@ -308,6 +308,15 @@ 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