Fixed #19310 -- changed method docs formatting for custom file storage docs

                    option = settings.CUSTOM_STORAGE_OPTIONS

                ...

#. Your storage class must implement the ``_open()`` and ``_save()`` methods,

   along with any other methods appropriate to your storage class. See below for

   more on these methods.

#. Your storage class must implement the :meth:`_open()` and :meth:`_save()

   methods, along with any other methods appropriate to your storage class. See

   below for more on these methods.

   In addition, if your class provides local file storage, it must override

   the ``path()`` method.

You'll also usually want to use hooks specifically designed for custom storage

objects. These are:

``_open(name, mode='rb')``

~~~~~~~~~~~~~~~~~~~~~~~~~~

.. method:: _open(name, mode='rb')

**Required**.

you'll want to return some subclass here that implements logic specific to the

backend storage system.

``_save(name, content)``

~~~~~~~~~~~~~~~~~~~~~~~~

.. method:: _save(name, content)

Called by ``Storage.save()``. The ``name`` will already have gone through

``get_valid_name()`` and ``get_available_name()``, and the ``content`` will be a

passed in, but if the storage needs to change the file name return the new name

instead).

``get_valid_name(name)``

~~~~~~~~~~~~~~~~~~~~~~~~

.. method:: get_valid_name(name)

Returns a filename suitable for use with the underlying storage system. The

``name`` argument passed to this method is the original filename sent to the

The code provided on ``Storage`` retains only alpha-numeric characters, periods

and underscores from the original filename, removing everything else.

``get_available_name(name)``

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. method:: get_available_name(name)

Returns a filename that is available in the storage mechanism, possibly taking

the provided filename into account. The ``name`` argument passed to this method