Add small note for handoffs_only acceleration
Add small note for handoffs_only acceleration

The related change benefitts us greatly with decreasing hash
calculations for ignored partitions when handoffs_only mode is on
however, at the same time, it has been not able to care misplaced
fragments in the primary partition so that add the notation into the
code to let us know as "this is by design".

Change-Id: I16e3c1170af710e2a6ecc0b7267ab369450095bf
Related-Change: 44c63c69909e532e383017e7c900e4b7d13e0165

openstack/swift,clayg/swift,matthewoliver/swift,clayg/swift,matthewoliver/swift,tipabu/swift,nadeemsyed/swift,notmyname/swift,notmyname/swift,nadeemsyed/swift,matthewoliver/swift,nadeemsyed/swift,tipabu/swift,nadeemsyed/swift,swiftstack/swift,notmyname/swift,smerritt/swift,openstack/swift,notmyname/swift,smerritt/swift,psachin/swift,swiftstack/swift,openstack/swift,clayg/swift,smerritt/swift,tipabu/swift,swiftstack/swift,matthewoliver/swift,psachin/swift,openstack/swift,psachin/swift,clayg/swift,tipabu/swift,psachin/swift,smerritt/swift