archive/tar: document Header fields and Type flags
archive/tar: document Header fields and Type flags

Documentation more along the lines of os.FileInfo
Fixes issue 2180.

R=golang-dev, dsymonds
CC=golang-dev, mike.rosset
http://codereview.appspot.com/4958055

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go