remotefs: Fix doc string for _create_snapshot
remotefs: Fix doc string for _create_snapshot

Fix the documentation string for _create_snapshot() function.
Added detailed doc for volume in-use and available scenarios for the
snapshot deletion cases.

Tested-By: Bharat K Kobagana <2fc3c50d40aa5bb966d1303f3150edbad1c3ee0c@redhat.com>
Change-Id: I60b83640c49f5ba9cc36aac950d8d193c06a900d

bswartz/cinder,phenoxim/cinder,nikesh-mahalka/cinder,JioCloud/cinder,apporc/cinder,JioCloud/cinder,openstack/cinder,tlakshman26/cinder-https-changes,nexusriot/cinder,petrutlucian94/cinder,cloudbase/cinder,apporc/cinder,CloudServer/cinder,manojhirway/ExistingImagesOnNFS,duhzecca/cinder,Paul-Ezell/cinder-1,tlakshman26/cinder-bug-fix-volume-conversion-full,tlakshman26/cinder-new-branch,Paul-Ezell/cinder-1,NetApp/cinder,Hybrid-Cloud/cinder,nikesh-mahalka/cinder,bswartz/cinder,winndows/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,tlakshman26/cinder-https-changes,NetApp/cinder,leilihh/cinder,j-griffith/cinder,eharney/cinder,scottdangelo/RemoveVolumeMangerLocks,sasukeh/cinder,scality/cinder,potsmaster/cinder,ge0rgi/cinder,takeshineshiro/cinder,dims/cinder,takeshineshiro/cinder,cloudbase/cinder,openstack/cinder,Hybrid-Cloud/cinder,duhzecca/cinder,j-griffith/cinder,mahak/cinder,dims/cinder,scottdangelo/RemoveVolumeMangerLocks,saeki-masaki/cinder,petrutlucian94/cinder,sasukeh/cinder,Datera/cinder,manojhirway/ExistingImagesOnNFS,saeki-masaki/cinder,winndows/cinder,mahak/cinder,Nexenta/cinder,nexusriot/cinder,potsmaster/cinder,tlakshman26/cinder-new-branch,scality/cinder,CloudServer/cinder,Nexenta/cinder,phenoxim/cinder,Datera/cinder,leilihh/cinder,eharney/cinder