block_types: add comments about Holes
block_types: add comments about Holes

Suggested by @akalin-keybase.

Issue: #531

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