Merge pull request #4599 from ipfs/feat/doc-interfaces

blocks/blockstore/blockstore.go

blockservice/blockservice.go

core/builder.go

core/core.go

core/coreapi/coreapi.go

core/coreapi/dag.go

core/coreapi/interface/interface.go

core/coreapi/key.go

core/coreapi/name.go

core/coreapi/unixfs.go

core/coreunix/add.go

keystore/keystore.go

keystore/memkeystore.go

merkledag/merkledag.go

mfs/system.go

pin/pin.go

repo/repo.go