Fixed #24510 -- Clarified FileField.upload_to docs.

.. attribute:: FileField.upload_to

    A local filesystem path that will be appended to your :setting:`MEDIA_ROOT`

    setting to determine the value of the

    :attr:`~django.db.models.fields.files.FieldFile.url` attribute.

    This attribute provides a way of setting the upload directory and file name,

    and can be set in two ways. In both cases, the value is passed to the

    :meth:`Storage.save() <django.core.files.storage.Storage.save>` method.

    This path may contain :func:`~time.strftime` formatting, which will be

    replaced by the date/time of the file upload (so that uploaded files don't

    fill up the given directory).

    If you specify a string value, it may contain :func:`~time.strftime`

    formatting, which will be replaced by the date/time of the file upload (so

    that uploaded files don't fill up the given directory). For example::

    This may also be a callable, such as a function, which will be called to

    obtain the upload path, including the filename. This callable must be able

    to accept two arguments, and return a Unix-style path (with forward slashes)

    to be passed along to the storage system. The two arguments that will be

    passed are:

        class MyModel(models.Model):

            # file will be uploaded to MEDIA_ROOT/uploads

            upload = models.FileField(upload_to='uploads/')

            # or...

            # file will be saved to MEDIA_ROOT/uploads/2015/01/30

            upload = models.FileField(upload_to='uploads/%Y/%m/%d/')

    If you are using the default

    :class:`~django.core.files.storage.FileSystemStorage`, the string value

    will be appended to your :setting:`MEDIA_ROOT` path to form the location on

    the local filesystem where uploaded files will be stored. If you are using

    a different storage, check that storage's documentation to see how it

    handles ``upload_to``.

    ``upload_to`` may also be a callable, such as a function. This will be

    called to obtain the upload path, including the filename. This callable must

    accept two arguments and return a Unix-style path (with forward slashes)

    to be passed along to the storage system. The two arguments are:

    ======================  ===============================================

    Argument                Description

                            when determining the final destination path.

    ======================  ===============================================

    For example::

        def user_directory_path(instance, filename):

            # file will be uploaded to MEDIA_ROOT/user_<id>/<filename>

            return 'user_{0}/{1}'.format(instance.user.id, filename)

        class MyModel(models.Model):

            upload = models.FileField(upload_to=user_directory_path)

.. attribute:: FileField.storage

    A storage object, which handles the storage and retrieval of your