[1.8.x] Fixed #24665 -- Clarified model field flag defaults.

Backport of 2b086229 from master

docs/ref/models/fields.txt

@@ -1912,7 +1912,7 @@ Attributes for fields with relations
 
 These attributes are used to query for the cardinality and other details of a
 relation. These attribute are present on all fields; however, they will only
-have meaningful values if the field is a relation type
+have boolean values (rather than ``None``) if the field is a relation type
 (:attr:`Field.is_relation=True <Field.is_relation>`).
 
 .. attribute:: Field.many_to_many