--pkg -> --package-root (dartdoc)

Staged at:

http://pkg.dart-lang.appspot.com/docs/dart-up-and-running/contents/ch04-tools-dartdoc.html
http://pkg.dart-lang.appspot.com/docs/dart-up-and-running/contents/preface.html#preface-updates-v3

Related CL: https://chromiumcodereview.appspot.com/17319002/

Review URL: https://chromiumcodereview.appspot.com/17313002