Add missing new lines on docs

issue10887
review387211002
(grafted from 71e538c0e6ea1f94198927715e0da1d25e908fa5)

--HG--
branch : 6.0

doc/ref/models/models.rst

@@ -42,13 +42,16 @@ Class attributes are:
     into the queue.
 
 .. attribute:: Model._fields
+
     It contains a dictionary with the field name as key and its
     :class:`~trytond.model.field` instance as value.
 
 .. attribute:: Model._record
+
     It stores the record class to store internaly the values of the instances.
 
 .. attribute:: Model._defaults
+
     It contains a dictionary with the field name as key and its default method
     as value.