archive/tar: document Header fields and Type flags

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

R=golang-dev, dsymonds
CC=golang-dev, mike.rosset
https://golang.org/cl/4958055

src/pkg/archive/tar/common.go

@@ -15,36 +15,37 @@ const (
 	blockSize = 512
 
 	// Types
-	TypeReg           = '0'
-	TypeRegA          = '\x00'
-	TypeLink          = '1'
-	TypeSymlink       = '2'
-	TypeChar          = '3'
-	TypeBlock         = '4'
-	TypeDir           = '5'
-	TypeFifo          = '6'
-	TypeCont          = '7'
-	TypeXHeader       = 'x'
-	TypeXGlobalHeader = 'g'
+	TypeReg           = '0'    // regular file.
+	TypeRegA          = '\x00' // regular file.
+	TypeLink          = '1'    // hard link.
+	TypeSymlink       = '2'    // symbolic link.
+	TypeChar          = '3'    // character device node.
+	TypeBlock         = '4'    // block device node.
+	TypeDir           = '5'    // directory.
+	TypeFifo          = '6'    // fifo node.
+	TypeCont          = '7'    // reserved.
+	TypeXHeader       = 'x'    // extended header.
+	TypeXGlobalHeader = 'g'    // global extended header.
 )
 
 // A Header represents a single header in a tar archive.
 // Some fields may not be populated.
 type Header struct {
-	Name     string
-	Mode     int64
-	Uid      int
-	Gid      int
-	Size     int64
-	Mtime    int64
-	Typeflag byte
-	Linkname string
-	Uname    string
-	Gname    string
-	Devmajor int64
-	Devminor int64
-	Atime    int64
-	Ctime    int64
+	Name     string // name of header file entry.
+	Mode     int64  // permission and mode bits.
+	Uid      int    // user id of owner.
+	Gid      int    // group id of owner.
+	Size     int64  // length in bytes.
+	Mtime    int64  // modified time; seconds since epoch.
+	Typeflag byte   // type of header entry.
+	Linkname string // target name of link.
+	Uname    string // user name of owner.
+	Gname    string // group name of owner.
+	Devmajor int64  // major number of character or block device.
+	Devminor int64  // minor number of character or block device.
+	Atime    int64  // access time; seconds since epoch.
+	Ctime    int64  // status change time; seconds since epoch.
+
 }
 
 var zeroBlock = make([]byte, blockSize)