- separated exported data structures from doc reader
  by extracting all exported data structures into doc.go
  and moving the implementation into reader.go
- added missing documentation comments
- no API or semantic changes (but moved positions of
  PackageDoc.Doc and TypeDoc.Decl field up for consistency)
- runs all tests

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5527063