Improve documentation of default argument for fields

The documentation now not only mentions that callables are acceptable
arguments, it also tells the reader what the callable should look like.

docs/api-guide/fields.md

@@ -51,7 +51,7 @@ Defaults to `False`
 
 If set, this gives the default value that will be used for the field if no input value is supplied.  If not set the default behavior is to not populate the attribute at all.
 
-May be set to a function or other callable, in which case the value will be evaluated each time it is used.
+May be set to a function or other callable, in which case the value will be evaluated each time it is used. When called, it will receive no arguments. If the callable has a `set_context` method, that will be called each time before getting the value with the field instance as only argument.
 
 Note that setting a `default` value implies that the field is not required. Including both the `default` and `required` keyword arguments is invalid and will raise an error.