folder_block_manager: comments suggested by @akalin-keybase
folder_block_manager: comments suggested by @akalin-keybase

Issue: #857

keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client