Improve examples for X-Sendfile

[benoitbryon]

In documentation:

• examples for X-Sendfile are not clear (include directive references wrong code)
• what URL middlewares catches and what URL it passes as X-Senfile header should be explained.

As an example:

• given settings.MEDIA_URL = '/media/'
• given a model with file = models.FileField(upload_to='documents'). It will have file.url prefixed with MEDIA_URL and UPLOAD_TO, i.e. /media/documents/ in our example.
• given source_url is "/media/documents/", middleware will catch DownloadResponse where file.url starts with "/media/documents/", such as "/media/documents/sample-filename.pdf" => middleware will send "/sample-filename.pdf" to x-sendfile
• given destination_url is "/var/lib/media/documents" (notice no trailing backslash) => x-sendfile value will be /var/lib/media/documents/sample-filename.txt
• in general, destination_url should match one value configured as XSendFilePath in Apache configuration.