Add comment about ResumingGetter.used_source_etag
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This confused a couple developers and took about ten minutes to
unravel in IRC; let's leave a clue for the next person.

Change-Id: I356c8c7a44de23f02eaf68d23a39c9eb4c203ff1

hbhdytf/mac2,tipabu/swift,redhat-openstack/swift,redbo/swift,clayg/swift,tipabu/swift,openstack/swift,hbhdytf/mac2,nadeemsyed/swift,swiftstack/swift,thiagodasilva/swift,levythu/swift,shibaniahegde/OpenStak_swift,revoer/keystone-8.0.0,redhat-openstack/swift,rackerlabs/swift,hbhdytf/mac2,psachin/swift,psachin/swift,tipabu/swift,smerritt/swift,notmyname/swift,hbhdytf/mac2,notmyname/swift,shibaniahegde/OpenStak_swift,aerwin3/swift,openstack/swift,nadeemsyed/swift,nadeemsyed/swift,prashanthpai/swift,williamthegrey/swift,daasbank/swift,levythu/swift,tipabu/swift,rackerlabs/swift,takeshineshiro/swift,clayg/swift,smerritt/swift,bouncestorage/swift,psachin/swift,clayg/swift,hbhdytf/mac,notmyname/swift,gold3bear/swift,openstack/swift,bradleypj823/swift,revoer/keystone-8.0.0,larsbutler/swift,bradleypj823/swift,clayg/swift,bouncestorage/swift,bkolli/swift,matthewoliver/swift,matthewoliver/swift,larsbutler/swift,smerritt/swift,hurricanerix/swift,hurricanerix/swift,matthewoliver/swift,openstack/swift,mjzmjz/swift,swiftstack/swift,hurricanerix/swift,matthewoliver/swift,redbo/swift,Akanoa/swift,psachin/swift,Em-Pan/swift,dencaval/swift,aerwin3/swift,mjwtom/swift,mjzmjz/swift,williamthegrey/swift,Akanoa/swift,hurricanerix/swift,notmyname/swift,gold3bear/swift,daasbank/swift,nadeemsyed/swift,bkolli/swift,dencaval/swift,prashanthpai/swift,smerritt/swift,thiagodasilva/swift,swiftstack/swift,takeshineshiro/swift,Em-Pan/swift,mjwtom/swift,hbhdytf/mac