[1.7.x] Fixed #22618 -- Improved import_string example.

    example::

        from django.utils.module_loading import import_string

        ImproperlyConfigured = import_string('django.core.exceptions.ImproperlyConfigured')

        ValidationError = import_string('django.core.exceptions.ValidationError')

    is equivalent to::

        from django.core.exceptions import ImproperlyConfigured

        from django.core.exceptions import ValidationError

.. function:: import_by_path(dotted_path, error_prefix='')