Fixed #26508 -- Clarified docs for various FieldFile methods.

        The underlying :py:term:`file object` that this class wraps.

        .. admonition:: Be careful with this attribute in subclasses.

            Some subclasses of :class:`File`, including

            :class:`~django.core.files.base.ContentFile` and

            :class:`~django.db.models.fields.files.FieldFile`, may replace this

            attribute with an object other than a Python :py:term:`file object`.

            In these cases, this attribute may itself be a :class:`File`

            subclass (and not necessarily the same subclass). Whenever

            possible, use the attributes and methods of the subclass itself

            rather than the those of the subclass's ``file`` attribute.

    .. attribute:: mode

        The read/write mode for the file.

When you access a :class:`~django.db.models.FileField` on a model, you are

given an instance of :class:`FieldFile` as a proxy for accessing the underlying

file. In addition to the functionality inherited from

:class:`django.core.files.File`, this class has several attributes and methods

that can be used to interact with file data:

file.

The API of :class:`FieldFile` mirrors that of :class:`~django.core.files.File`,

with one key difference: *The object wrapped by the class is not necessarily a

wrapper around Python's built-in file object.* Instead, it is a wrapper around

the result of the :attr:`Storage.open()<django.core.files.storage.Storage.open>`

method, which may be a :class:`~django.core.files.File` object, or it may be a

custom storage's implementation of the :class:`~django.core.files.File` API.

In addition to the API inherited from

:class:`~django.core.files.File` such as :meth:`~django.core.files.File.read`

and :meth:`~django.core.files.File.write`, :class:`FieldFile` includes several

methods that can be used to interact with the underlying file:

.. warning::

    Two methods of this class, :meth:`~FieldFile.save` and

    :meth:`~FieldFile.delete`, default to saving the model object of the

    associated ``FieldFile`` in the database.

.. attribute:: FieldFile.name

The name of the file including the relative path from the root of the

:class:`~django.core.files.storage.Storage` of the associated

:class:`~django.db.models.FileField`.

.. attribute:: FieldFile.size

The result of the underlying :attr:`Storage.size()

<django.core.files.storage.Storage.size>` method.

.. attribute:: FieldFile.url

.. method:: FieldFile.open(mode='rb')

Behaves like the standard Python ``open()`` method and opens the file

associated with this instance in the mode specified by ``mode``.

Opens or reopens the file associated with this instance in the specified

``mode``. Unlike the standard Python ``open()`` method, it doesn't return a

file descriptor.

Since the underlying file is opened implicitly when accessing it, it may be

unnecessary to call this method except to reset the pointer to the underlying

file or to change the ``mode``.

.. method:: FieldFile.close()