[1.5.x] Documentation - Noted that OneToOneField doesn't respect unique.

Added OneToOneField to the list of model fields for which the unique
argument isn't valid. (OneToOneFields are inherently unique, and if
the user supplies a value for unique it is ignored / overwritten.)
Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.