[3.1.x] Improved ManyToManyField.through docs.

Backport of e7fa8aff43 from master

docs/ref/models/fields.txt

@@ -1837,6 +1837,13 @@ that control how the relationship functions.
     :ref:`extra data with a many-to-many relationship
     <intermediary-manytomany>`.
 
+    .. note::
+
+        If you don't want multiple associations between the same instances, add
+        a :class:`~django.db.models.UniqueConstraint` including the from and to
+        fields. Django's automatically generated many-to-many tables include
+        such a constraint.
+
     .. note::
 
         Recursive relationships using an intermediary model and defined as
@@ -1870,7 +1877,9 @@ that control how the relationship functions.
       points (i.e. the target model instance).
 
     This class can be used to query associated records for a given model
-    instance like a normal model.
+    instance like a normal model::
+
+        Model.m2mfield.through.objects.all()
 
 .. attribute:: ManyToManyField.through_fields