write a few package doc strings to improve look of godoc

blocks/blocks.go

blocks/bloom/filter.go

blocks/set/set.go

blockservice/blockservice.go

blockstore/blockstore.go

config/config.go

crypto/key.go

crypto/spipe/handshake.go

diagnostics/diag.go

fuse/ipns/ipns_unix.go

fuse/readonly/readonly_unix.go

importer/chunk/splitting.go

importer/importer.go

merkledag/merkledag.go

path/path.go

peer/peer.go

routing/dht/dht.go

routing/kbucket/bucket.go

routing/kbucket/table.go

routing/kbucket/table_test.go