Update docs to be more clear about nested classes (refs #23418)

- ``datetime.date``, ``datetime.time``, and ``datetime.datetime`` instances

- ``decimal.Decimal`` instances

- Any Django field

- Any function or method reference (e.g. ``datetime.datetime.today``)

- Any class reference

- Any function or method reference (e.g. ``datetime.datetime.today``) (must be in module's top-level scope)

- Any class reference (must be in module's top-level scope)

- Anything with a custom ``deconstruct()`` method (:ref:`see below <custom-deconstruct-method>`)

Django can serialize the following on Python 3 only:

Django cannot serialize:

- Nested classes

- Arbitrary class instances (e.g. ``MyClass(4.3, 5.7)``)

- Lambdas