use_multipath_for_image_xfer and enforce_multipath_for_image_xfer need to be
used for each backend section or in [backend_defaults] section in cinder.conf
using it in default section will not have any effect.
This patch fixes multiple driver doc regarding use_multipath_for_image_xfer
and enforce_multipath_for_image_xfer parameter
also added same help text in driver.py so that the sample config file
indicates the same.
Change-Id: I5c9b3e5142833f276e10c432a7705a8cd118be7c
Closes-Bug: 1856612